Like Stavros said while I was typing.......Fitting a speaker into a boat is not quite like designing a speaker cabinet for home use, but some of the principals are still there.
Sound comes off the back as well as the front of the cone. If allowed to mix with the front sound, they will cancel out, so you need to separate the two. Having one with free access to the rest of the world (so you can hear it) and the other enclosed (so you can't) works. So does having a long air path between the two. The deepness of the sound that the speaker can produce depends, among other things, on the longest dimension of the rear enclosure.
On a boat, this usually means finding a flat surface that you can make a big enough hole in so that the hull becomes the enclosure and arranging to hide the speaker cone with something that doesn't muffle the sound.
Deeper sounds often means bigger speakers, not always feasible in a model. Small speakers can handle low frequencies, but do not shift the same quantity of air for the same travel. This means that the smaller cone has to travel further to be as loud. This in turn means a lot more engineering in the quality of the build. More solidly built, more weight, costs more and a monstrous magnet, and costs more. Speaker sizes are often quoted in diameter of the cone, not the chassis diameter, a point to watch out for.
The cones are normally made of paper, and, as one side needs to be exposed to probably damp air, needs protection. Putting the speaker into a plastic bag before mounting keeps water away from the paper, but lets the sound out. There are mylar cone speakers, but I haven't seen any yet that would do a good job of handling motor sounds on a 1:12 lifeboat.