If you want to have some bass, but not putting a subwoofer in the boat.. then I would recommend the Alpine 7" Marine Speakers as they have great low frequency response... this will require a little cutting to install the speakers.. frankly all of the speaker manufacturers listed will be a great improvement, especially if you put in a small amplifier.. on the amplifier side I would highly recommend Alpine as they are digital "Class D type" super efficient from a power consumption standpoint..
If you are going to install this yourself EBAY or online will likely get you the best prices on the gear
Best of luck,
Mac