The ESC's you quote do not have a BEC output, so that has nothing to do with your problem, and Malcolm is right (as always) that the crystal would not be affected by anything happenning at the output end of the system. How do you know the crystal has failerd - I can only assume that you have had the set working OK by changing to another pair of crystals ?
I have the same speed controllers in a Smit Nederland tug running 2 Buehler motors, and have just been and tried running both ESC's through a Y lead to the Rx and had no problem, both ESC's ran, crystals are still ok - radio set used was a Futaba Attack 2 channel.
I'm sure Laurie and Jackie (model slipway) will be able to recommended suitable motors for the boat if you email them via their web site, and if you checkout Model Power at
http://www.modelpower.co.uk/acatalog/Trent_Class_Life_Boat.html they advertise a battery pack specifically aimed at the Trent.
Good luck
Aston