what size wheels you spinning, must be loading the engine pretty hard, you might try trimming up a bit to aerate the prop some

And speaking of IAC's, if you don't have fuses on them, do it this week. You'll save your ECU's when the IACs fail. Long thread here on OSO with pics, info, etc. Search my posts.

Then keep a couple of spare fuses and an IAC in the boat to save your day/weekend.

^^^^^
Merc 600 SCI IAC 1 Amp Fuse Install