The tweeters in the Clarion units are notorious for failing; as is the entire speaker after a few seasons - Hence why MC have now upgraded to the JBL Components (Think this happened mid 2004 so yours could have either - if they are JBL I'd be far more surprised if they've failed) - Moving to JBL was a great move on MC's part.
The speakers you have (I believe Clarion) will be 6.5" and ideally need to be replaced by a decent quality Marine speaker (especially as you are a salt water boater). You can go with the new style JBL units but you'll need to cut holes for the seperate tweeter.. or go to something like the Kicker KM60's - This will be a drop in solution and provide far better sound than the Stock speakers you have ever could.
I stock the Kicker Speakers -
Wakeboard Towers UK Homepage / -01270 872161, or if you want the JBL units you'll want to chat to Carl at MasterCraft Parts UK - 01977 631 699.
Alternatively hit Ebay or your local JBL audio Stockist - They should be able to order the JBL marine stuff. IF you want Kicker Marine you'll need to work via myself or a Wakeboard Towers UK dealer (unless someone has old stock laying around).
If you need any advice (whether I supply or not) feel free to give me a call in the office (01270 872161) - or if you need fitting at Menai I can do it when I'm next over there (was there this aft on the Jetski, will likely be around again within 2 weeks) in exchange for a couple of cold beers
Hope that helps a bit
Mike
Wakeboard Towers UK