Of my own experience, (My first job was seller/mounting hifi in cars), Infinity made in the past real HIFI speakers/amps. With the kappa (green speakers, tweeter EMIT etc.) and BETA speakers.
My brother recently bought a Reference 2 ohms Infinity kit, I was realy frustrated about the result. Sound was "acid", with omnipresent crying tweeters.
I don't know about your sub and speakers, take a look at the X-max and sensitivity. In a boat, you need pressure to hear music. Bet on high sensitivity speakers, HIFI style (Infinity is/was good IMO), with powerful US amplifiers. I would recommand Rockord, MTX, Orion, Polk, hifonics, kicker, Autotek, JL audio, Cerwin Vega, some Crunch are good.
Forget about japanese amplifiers, lack of power for a boat IMO.

You can go for a 4 channel amp to run 4x 6 1/2' (connected in parallel) + 1 SUB (4 ohms single coil or 2x 2 ohms). A mono amp for a Sub will be interresting in class-D for multi coil or multi sub application. For a low budget, prefer a good 4 channel class-D

Here is my setup connection :
2x channel MTX 62.5w/channel for [4 speakers // wirrered in the boat, that's 120Wrms on each, high pass filter @ 100 HZ]
4x channel ORION 75w/channel for [4 speakers // wirrered on the tower, that's 150Wrms on each, high pass filter @ 80 HZ] on channel 1/2 + [the subwoofre bridged, 300Wrms, low pass filter @ 80HZ] on channel 3/4

Note that my Pioneer unit has 3x RCA outputs @ 4V. Voltage is important as it gives more dynamics and power. Most of low prices CD players are 2V ...