With brush type motors, and a "proper" marine type ESC, reverse is normal.  When you look at car ESCs, you get braking, which you dont need on a boat, and reverse might need a procedure to go through.  This applies with most of the allegedly reversing brushless ESCs and motors.  Aircraft ESCs dont usually need reverse.  Since reverse is your brakes, you usually need it in a hurry, as the bank gets uncomfortably close to the front end of the boat.  NB, fitting the props using a properly fitted locknut plus some thread sealer is a GOOD THING, since chucking it in reverse generally causes the prop to try to unwind itself from the shaft.

With 2 ESCs, and a basic transmitter, you can fit a mixer to combine rudder and throttle channels to differentiate the signals to the ESCs.  With the higher end programmable TXs, the TX can do the mixing.

Connecting two rudder arms is very similar to the steering on your buggy, unless that has a servo for each front wheel.  It is possible to plug two servos into one channel via a "Y" lead, depending on the space arrangements in your boat, it might be the simplest way to go for two rudders.