anything is possible, however I have came to the same thump in the road.....pwc's require the best marine speakers anything less is a waste of money as they come apart fast do to the elements at hand in marine use where as a boat the speakers are not as exposed.
Most get an amp and 4 speaker system, your mp3 plugs right into the amp via a cable. The 4 speakers go where ever you want to cut a hole for them, I do not recommend the mirror speaker mounts as you really need those mirrors if you have them.
Price wise I have seen them from $500 up to about $900 or so. Can you get away with a couple hundred?......yes but you will be replacing everything within couple of months.....mounting.....typical mounting is where ever you will hear the sound best....amp gets mounted inside, I would mount it out of dict water contact.