hi, be aware of accuracy issues though. A few thoughts based on industry experience with some guidance systems, if you had not already though of them):
the main issue is accuracy of measuring low values (ie. water speed vs. air speed). in general it is easier to get good accuracy when the signal you are measuring is medium/high. If speed measurment is not accurate then any dead reckoning will suffer, think of it in terms of how long the dead reckoning period is vs speed vs accuracy and see how far out you will be at the end of that period (before getting a good GPS fix again).
The other source of inaccuracy is the rate of turn, in an aircraft the rate of turn is quite low, so there will be a number of positiion estimates during the turn, but in a water craft the trun rate can be much higher, and with a greater potential for drifting round the corner (side slip) so the simple integration of speed & direction no longer give the accurate change in position.
Going back to sensors, 'good' industrial flow rate (speed) senors use ultrasonic dopler, but not sure if you could get a cheap/small sensor, so a pitot/pressure is probably the best, though calibration could be 'interesting'!
Another alternative is a small vane/paddle sticking out of the hull into the (smooth) water flow with a strain guage measuring the force on the paddle/vane. Probably not linear force vs speed.