Here's how i'd do it
I would be tempted to 3 receivers and 3 radio switches from action or robbe, operating on a BEC, say from mtroniks viper esc's with a radio microswitch from the master esc on-off switchon the battery side of each one. so when in up, the the master RX would be told to power up the land drive, when you hit the water flick the switch down to turn off the road drive and break the electrical connection between the esc and wheels, this is to prevent a dynamo action potentially damaging the esc. it would also through the other switch power up the wet drive
another (much cheaper) option would be to have two esc's and a switch, the two esc's would share a common negative, and the radio switch would divert power to either the road transmission esc or the maritime drive esc. the switches have a 3 pin block and the metal reed flicks betweem 1+2 and 2+3, so...
in pin 2 you have you red wire from the battery,
in pin 1 is the wire to the road esc,
in pin 3 is the wire to the maritime drive esc
in the down position, the switch would be joining 1+2, so 3 would be dead
in the up position shw switch would be joining 2+3, so 1 would be dead.
for unsurance you could put a second relay between the esc for the wheels motor, and the wheels motor, this is to prevent any possible damage from a dynamo action frying your esc, and esc's dont take kindly to power coming in from the motor end.