I have an 18 V Mastercraft (Canadian Tire) cordless I am quite pleased with, but it's really overkill for most boat jobs, so it stays at home with its companion cordles 5 1/2" circular saw (der Uberpruner). Aboard I use a wimpy 6V Black and Decker "electric screwdriver", which is, however, very good with the softer brass screws, and a 14 V Black and Decker 3/8" cordless drill that somebody in the club threw out! I cleaned it up, charged the TWO batteries and it works great!
But for the tough stuff, I have a 1/2" Makita hammer drill that runs off a cord and is just brutal, which is sometimes necessary on steel boats. I have a 4" Makita angle grinder aboard which is compact enough to get in tight spots, and a 7 1/2" Makita circular saw I love for cutting planking.
I'm pretty pleased with Makita, I'd have to say, although I wouldn't turn down some of the DeWalt and Milwaukee power tools I've seen. Strangely, considering it's a "house brand", my six amp Mastercraft reciprocating saw went through 30 or so blades cutting new portlight holes on my buddy's steel ketch, and is currently chewing through a tenacious bead of 5200 and some sheared bolts between a steel flange and my aluminum pilothouse roof. So I won't diss what is likely a Chinese knock-off, but maybe isn't because it's about six years old. It helped to tear down a shack a few years ago, and it's still going strong.