If you are looking for something capable of salt water use that implies something that is seaworthy. You can get boats surveyed prior to purchase. If you know less about boats than you do cars, then the survey would make sense. Fibre glass hulls do need proper examination. The grp can fail, or be damaged by impact, so don't assume just because the anitfouling is in good order that the grp is too.
Engines and their maintenance history will be important. Boat engines service intervals are measured in hours run, rather than mileage. You'll want to know the range of the boat and it's speeds. Boats for coastal use may be less suitable for river use due to their gearing and size of propellers relative to rudder. Many of the "gin palaces" on the Thames won't fit under bridges, and even in idle would exceed the speed limits.
If you know little about boats you may want to first get involved as a social member with a local club. You'll pick up a few bits of knowledge, and it may be that some owners will be looking for crew assistance from time to time. Some clubs may have boat handling type competitions whcih can be great to hone your skills so when you get your toy you are less likely to end up ramming the shore
Remember the adage. "A boat is a hole in the water into which you pour money." Buying it is one thing. Think of the maintenance costs, mooring costs, and even the fuel. How much will it cost to put the kit on the boat you need, and that's not just lifejackets, flares and navigation aids, but it's also a lot of the basic living stuff like crockery and bedding. Can you afford to keep it once you have it?