What's up with my spark plugs???

I took my 1994 SeaDoo GTX to the beach with me for the first time last
weekend. I took it out to the ocean and on the way back to the ramp in the
waterway I noticed it was not running smooth. It would sort of bog out
every once in a while. The next day I rode it from North Myrtle Beach to
Holden Beach (if you are familiar with NC) and on the way back it would
barely run at all when we got to Sunset. We pulled over and beached it and
I took the spark plugs out to find brown stuff caked up on both of them,
bridging the gap. I figured they were just gone bad, so I had a friend
bring me another set. We put the new ones on and it ran like a charm. We
rode back to N Myrtle and went out in the ocen again and on the way back it
did the same thing. This is only about 2 hours after I put brand new ones
in it. It would barely run at all. It was the spark plugs again. Has
anyone ever seen this before? Any ideas on what it could be?

Matt