I'm trying to think of any inexpensive dependable amps that have internal fans..... Last years PG Titaniums did, but they weren't inexpensive and they still go into termal protection when in tight quarters on a boat... Toby can vouch for that...
I always recommend putting fans on on anything going into a boat and that will bedriven hard.
Look at the:
JBL PX300.4 and it's BPX500.1 for the subs
Alpine MRVF340 and a MRPM350 for the sub
JLAudio E6450 (economy 6 channel)and a JL 2501 for the sub
DEI 250.4 and a 350D for the subs
That's the few off the top of my head that I can think of...