Sounds like you need to have a dedicated TX for the boat. I had a similar problem as I fly planes also. I bought a Hobby King HK6s mode 1 which is a basic 6CH but is easy to convert to twin throttles for boats (I have 2,- 1 for smaller planes and 1 for boats + a 6x, 9x and 2 9XRs). Makes life a whole lot simpler not having to use relays
and switches for motors
. The other TX is the Fly Sky 9X (Turnigy 9X, IMax 9X) which has more switches and functions (plus easy to find extra RXs outside HK,- 3ch, 4ch, 6ch,which suit) or if you want overkill, a TGY 9XR pro and JR module
, which has everything you will NEVER need on a boat . Maybe you could use a program to pulse lights, (I have it lowering the flaps on my Cub in 2 stages in small steps /pulses and slow stepless up for eg,) or perhaps have the center motor only coming in over 1/2 throttle through a separate RX,- options are endless.
I think the HK 6s is the best if you just need a 6CH or the 9X for more features. You can use the rotary switch (6s) through a Sky RC remote switch (eg) to turn on lights etc plus the toggle sw for another function. Just depends on the $$ really
Not sure if you are using Lipos, brushed or brushless setup but either way sounds like 2x 3s 3000mah 20c lipos should work, especially if your motors
are brushed 12v , also maybe try reducing the prop pitch or diam to take a bit of load off the battery/s. Depends obviously on a number of things such as space, motor type (no of poles, KV, size etc), boat weight, battery type and capacity/discharge rate etc. Lipos are by far the best option for powering models.