I'm currently running the in boat speakers off of a 35x4 Clarion CD player and my 4 tower speakers off of an old HiFonics 200 watt amp. All of my speakers are Polk DB651s.
I have an old Image Dynamics IDQ 10" subwoofer sitting around that I'd love to put in the boat and I'd also love to have an aux input for MP3 players and phones. I don't want to spend much money at all on this though.
I was thinking that a good way to accomplish the two goals above would be to get a 5 channel amp and an EQ. I could run the in boat speakers off of the front two channels, the tower speakers off of the rear two and obviously the subwoofer off of the sub channel.
Anyway, this amp is really appealing to my budget:
I'd pair that up with an inexpensive equalizer to give me my volume control and aux input:
I googled it a bit and it seems to have a decent reputation among budget car audio people.