Interesting project - what's it for? Are you building a full-scale R/C boat? If you are doing some kind of remote-control or unmanned project, then read on...
Something you didn't mention is your fail-safes; it seems to me that by using ordinary servos you could get into a situation where throttle is wide open, and/or turning is stuck - this could happen, for instance, if power somehow is disconnected from the servo. Do you have a way to account for this?
If not, a suggestion would be to monitor the feedback potentiometer on the servo, and as you command it to go to a position, monitor the values received to verify that they are actually moving to the correct position. If the values don't change (or change erratically), engage a relay or contactor (or something else) to immediately shut off the engine by grounding the ignition coil output.
You should also have an external way to engage this same system remotely.
All of this only really matters if the project is remotely operated, or you don't have any easy access to where the servos are in case of a failure...