Fluid Mechanics has chapters on biotech, weapons, vehicles, and hardware (the latter is a miscellany, including everything from fusion reactors to drugs, but despite that vast scope is still barely longer than quite lengthy vehicles chapter). Overall, it's fairly repetitive, with many items that are just minor variations on items that have already appeared. And while some are interesting, most are fairly obvious utility items, or standard transhuman-lite gizmos. And it's not surprising since more than a decade and a half has passed since the supplement came out, but all the information technology stuff is very dated (e.g. the high end cell phone/computer doesn't come with either a GPS or a camera).

Organizationally, it's poor. The header levels are difficult to distinguish, and the table of contents is even worse since it has exactly 4 entries (the 4 chapters). So if you want to know where something is in the book within less than 30 pages, or what subgroupings are within each chapter (like sensors or body armor), you'll have to flip through page by page. And the index is no help -- while bioplastics are the core material used to design nearly everything on Poseidon, from structures to vehicles and from guns to gadgets, and there are at least 4 entries that focus on bioplastics, neither "bioplastic" nor "plastic" appears in the index. You literally have to know the specific name of an entry to find anything. These combine to make it a chore to find anything, again. Also, there are a few editing gaffs like missing values, bold formatting spilling over several extra lines, and the like.

The items use a standardized format, which is a few descriptive paragraphs followed by a standard list of stats. Which is a very odd (and poor) organizational choice, because there are stat blocks for specific models alongside stat blocks for huge groups or general concepts. And a lot of this would have been much easier to read and compare if it was presented in a general narrative, or in tables. (While there is a table for weapons, it doesn't help much. It's not just that it's in the back of the book, far far away from the writeups that explain what the weapons are and how they work; it's also very poorly laid out. Guiding lines or shading, for instance, would make it a lot easier to figure out which damage rating goes with which weapon.) And not all the characteristics make sense when applied to every item -- durability, for instance, is a measure of an object's toughness. You roll against it, to see if your toy is damaged. But whether it's potentially damaged in the first place is based on the DM arbitrarily deciding if the object could be damaged in the first place. So unless you're comparing two very similar items (it makes sense for vehicles and firearms, for instance), it's a completely subjective decision hidden behind the pretense of a mechanic.

It would also help if the stat blocks were tweaked for a wider variety of items (weapons and vehicles get a few extra lines, but everything else is identical). For instance, power output is listed for nearly all the power supplies, so why bury it in the text instead of giving it its own line? The stats also seem very inconsistent, going from obsessive detail in one entry, to looser descriptions in the next. In addition, the stats are often presented as a range, without any guidance for why the bigger or more expensive item is more desirable, and the descriptions often mention things that should clearly be game effects, but are never turned into mechanics. Different entries also have different information under the same stat. For instance, range is in kilometers for some vessels and hours for others, and dimensions sometimes includes mass (or is it displacement volume?) as well as length. There are also numerous small errors or apparent errors that make it hard to trust the numbers. For instance, 1 m^2 of solar panel weighs 0.5 kg... or does it weight 0.2 kg? (The stat block and description are in conflict.) Or a sub with a 35,000 ton weight (or displacement?) is supposed to be a heavy commercial hauler, but only has 8 metric tons of cargo capacity (another similar not entirely incomparable vehicle has a cargo capacity of 2,000 tons). This is contrasted with some obsessive numbers geekery when it comes to the interstellar spacecraft, where they provide specific numbers for power output, fuel usage rate, power densities, ship mass, cargo mass, and acceleration/deceleration time (and Gs) just to show the math behind the trip times. But curiously, none of that's statted out; it's just an essay (and all the other spacecraft do have stats). Other elements are very hand wavy, like the way improvements to vehicle armor costs the same whether you're buying it for a tiny one-person vessel or the most voluminous of all vehicles, and has no weight. Which is in contrast with ablative armor, which despite being limited by the the point value of the regular armor underneath it, has a weight and is purchased based on the surface area of the vehicle (even though no vehicles are given a surface area). Ultimately, the whole thing feels chaotic and incomplete.

The paragraph or three of descriptive text for each item tends to be the best part of the book, and is often fairly interesting, diving into technical explanations. But while doing this, they often forget to explain what the object is, upfront, so it takes a second reading to figure out what all that detail was referring to. And a lot of descriptions seem to more about building the setting than about the specific object being described, so much of that material would be better in sections giving overviews of the various types of technology that are available. The names of items are often colorful, though while some are easy to remember as well as evocative ("baggies" for the gel packs dolphins use to ingest drugs is great), others are needless obtuse or obscure with names that are more clever than descriptive, and thus will hard for players to remember amidst the tsunami of setting material they already have to remember. (E.g. "rigging value" has nothing to do with sailing, it's how hard an item is to modify, repair, or hack; while a "juicer" is a solvent, not any kind of drug or enhancement; and the flashy name "matador" is applied to, of all things, a stealth suit.) On top of that, a large number of the entries are pretty mundane. If they replaced the spread out stat blocks with tables or other more efficient formats, and trimmed back the more utilitarian entries (a cargo pod towed by a dolphin takes up more space than some combat vehicles, or a nanopaste that makes anything organic edible) and items that are unlikely to be used by PCs (like orbital cargo lifters), it wouldn't be hard to halve or even quarter the page count.

tl;dr A few gems, but a lot that's unexciting, the rules are wildly inconsistent, and it's poorly presented.