Both the listed motors are 1250kv, the max RPM is the same for a given voltage.
Motor wattage doesn't tell you anything about RPM, and a 600w motor isn't necessarily going to draw 600w in normal usage. If it's only lightly loaded it'll draw much less in the way of watts, but if the impeller is fouled, obviously a big wattage motor is going to cause more havoc than a low wattage one.
I'm not sure the GPS is going to have a tight enough resolution - to get an accuracy of <5cm on a ££££££ sidescan sonar setup we were trialling in work a while back, we were using 2 fairly large GPS heads with a metre seperation, as seen here;