Lots of things affect the radio reception to your sub, although antenna routing is important, it isn't hyper critical, and the very short range you are getting suggest some other issue, which could be with the sub itself, or the water you are sailing it in. Could also be a poor transmitter. Lots of variables here, and I'm afraid it's a case of a process of elimination to find out the cause- very difficult to diagnose by proxy.
A few things it could be, and ways to test (assuming you have spare equipment or can borrow a spare).
High mineral content water or brackish (salt content) water. Try another lake/pond/pool
Duff receiver. Try another receiver.
Duff BEC or ESC. Swap it out.
Duff TX. As above.
Duff crystals. Swap out. Irrelevant if using synthesized radio.
Bad wiring. Renew or rework.
Poor or no suppression on drive motor (unlikely but possible). Check motor has at least three suppression caps fitted.
Okay that's all the points i can think of. The Seawolf is a fairly straightforward boat, so shouldn't present too many problems on the electrical front.