Must admit, I'm with you on this Craig. Get the radio box as watertight as possible. Especially a leak like this, where it would have been coming in all the time the boat is running. Even if it's only a tiny amount, it doesn't take much, and obviously the longer it's running, the more there will be.
On the other hand, even when you've got a watertight radio box, things can go wrong, a split seal, or if you're like me, throw it in with the radio box bung missing, and don't even realise what you've done.
In the nitro boats, I'm still using 40MHz PWM radio, and as belt and braces, I like the failsafe to be separate from the receiver, and waterproof in its own right. Having a receiver with built in failsafe, in my opinion, is not as robust. If the all in one Rx / Failsafe gets wet, you can lose the receiver and failsafe functions at the same time, or lose the failsafe function first unknowingly, and subsequently lose the receiver function, and it can just stop working with the servos in their last position (like not having a failsafe at all, or sudden loss of power). On the basis that servos are usually more waterproof than receivers, if the receiver gets wet, and the failsafe is still working because it's waterproof, the failsafe will usually detect lost signal - due to wet receiver, and if the throttle servo is still working, shut the engine down.