As long as the main battery is giving more than 5 volts, the BEC will provide a regulated 5 volts for the receiver and any servos plugged into it. The instructions for the RX don't assume that an ESC with a BEC fitted will be used, so generic information only is supplied. They probably don't even consider use in a boat.
The kit instructions probably specify a particular battery, probably a Robbe part number. A quick check should show that it is a generic type of SLA, and any battery of the same voltage and capacity will weigh the same, have the same dimensions and do the job, but almost certainly at lower cost. Much the same applies with motors, as well.