I was thinking of adding 1 amp and either 1 or 2 subs to my boat and just curious if there is anything I should know. I saw on a website they sell open air subs, would these be better for in a boat? Would any type of car amp and sub work or should i get something different. I am not looking to spend much and I am looking for any input I can get.

Don't concern yourself with getting "marine" audio gear. You'll just spend more money for nearly the same stuff. Car audio gear will work just fine. <BR> <BR>I'd lean away from open air subs.

Does open air sub mean no box needed or is it just for like if it isnt in an enclosed space like a trunk or something because I am still planning on building custom boxes just dont know much about the open air subs.

Correct, open air means no box is needed.

I MOUNTED A KICKER SOLO BARIC UNDER THE STEERING WHEEL IN MY X2, IT HAD A CLARION FREE AIR BUT TOOK IT OUT AND PUT THIS SUB UNDER THERE IN A SEALED BOX AND THERE IS NO COMPARISON. THE SOLO BARIC SOUNDS 10 TIMES BETTER!

I am also curious if I even need 2 subs. I only have an 18ft I/O, would it be way too loud with 2 or what?

If you put a Kicker solo or a JL w6 or w7 under the dash in a proper box it will treat your nutz like a punching bag, you shouldn't need more than that...