Most cheap amplifiers do more harm than good. The THD (total harmonic distortion) is so high that they'll damage your speakers. A 40 watt RMS per channel amplifier is not as powerful as a typical head unit when the gain is turned to minimum (which is where the THD ratings are measured). THD on a head unit is measured at output without an adjustable gain. Too many people pick up a cheap amplifier, crank up the gain to get some volume (the perception was that an amplifier will make it louder) and wind up blowing their speakers.
Boats are a unique animal, because of all the extra noise (hull slap, waves, spray, motor, wind) ... the extra noise masks the distortion; our ears don't always hear it, but the speakers do, and the next thing you know you're sitting at anchor and want some quiet tunes and all you hear is crackle and buzz ...