For marine use, you likely need big power out.

One problem is the 12V supply at best gives you 12V peak-to-peak output. The RMS calculation for (sine) power would be (0.3535 * 12Vpp) squared divided by the load impedance (Erms ^ 2) / R). At 12V, you can't get a lot of power unless the speaker impedance is very low (say 2 Ohms) and the speaker is high efficiency like a horn device.

I think you could connect multiple speakers in parallel to create a lower impedance..

If you had two batteries and two amplifiers, you could bridge their connections to get 4 times the power to a single speaker...