If I recall Book of the Sea is a newer release than Seas of Blood (or vice versa if I'm wrong). Same company. Get the newer one.
I have Seafarer's handbook. It's alright. I rewrote the ship building rules, made a fancy spreadsheet to calculate it all based on SFH. It works. Got me pointed in the right direction.
Biggest thing to keep in mind is technology eras. I'm looking at 1600-1700's style ships (pirates of the caribean, Master and Commander). Most D&D references are 1400's or earlier. This mainly means smaller ships and therefore fewer weapons on deck.
Fewer weapons on deck may make things seem more playable (I got 3 heavy cats, you got 2 Heavy Cats and 2 ballistae). However, it isn't very realistic once you get to cannons. They're compact (maybe 5'x10' at most) and you can pack quite a few of them reasonably on a ship.
Janx