$2000 for manifolds and risers on a singe small block i/o? Sounds pretty crusty!
If you're at all handy it's not too hard a job and access should be pretty decent on your boat. You should be able to do it in a day even if it's your first time.
The big question of course is what damage did the water do in the cylinders? That could lead to all sorts of problems, up to complete engine replacement.
Step 1: retrieve boat from mechanic. he's taking you for a ride.
Step 2: check condition of oil - any signs of water?
Depending on how handy you are with a wrench, either do the following yourself, have a buddy give you a hand, or find a new mechanic to do the following:
Step 3: check compression. pay particular attention to the cylinders under the risers.
Step 4: If compression is good, replace manifolds, risers, and gaskets.
Where are you located? I'd bet somebody on the board could come lend you a hand if you had a cool refreshing beverage on stand-by one afternoon.
Don
'01 22 Classic, 502/B1
And a bunch of other stuff
"If everything seems under control, you're just not going fast enough" - Mario Andretti