As a former professional marine mechanic, I can attest to the value of having high-quality tools aboard. A combination wrench set (they're called combination because each wrench includes open and box ends) and socket kit fall squarely into that category. My preference is for strong yet thin wrenches. That's uncommon in run-of-the-mill tools; many manufacturers simply use more of a lower-quality alloy to attain the needed strength. It's worth spending a few extra dollars for thinner wrenches; there'll be times when only a skinny wrench will fit. Craftsman Professional Series Full Polish wrenches meet these requirements at an affordable price. The full-polish chrome effectively resists rust. Wrenches of this sort are available in 6- or 12-point series; my preference is for the latter. In tight spaces where you can only get an eighth of a turn, the 12-point carries the day. Today, most vessels have both metric and standard fasteners, which means a set of both are needed.
A socket set carries many of the same selection caveats: strong yet thin walls and 12-point configuration. A wide range of socket drive sizes is available. You should have 1/4- and 3/8-inch sets, and they should use a positive security mechanism for retaining the socket. That is, the socket shouldn't be able to separate from the wrench unless released by pushing a button. This prevents you from having to search for lost sockets in deep bilges.