1) plenty of quality build and good sounding marine built speakers out there with varying sound signature, grill designs and price points. Wet Sounds Revo and SX/SW-65i series, JL MX650, Kicker KM654LCW and Rockford MP262.

2) Is the reason for 4, looks? Id stick with an 8" or 10", regardless. Too many good options there, to go with a smaller 6.5". Plenty of options like the Wet Sounds Rev-10 or 8, Icon-8, JL 8.8, Kicker KM8, depending in goals. With the right setup, only a single pair might get the job done and come in less expensive then 4 pair of smaller speakers.

3) Not at opposed to two 10's if they can be done effectively. I would also consider a single 12" in a ported enclosure as an option.

4) once the final speaker options are decided, amp choices fall in line. Battery setup will be steered by use. As a minimum, id suggest a dual bank setup with at least one group 29/31 size deep cycle.