Hi Stuart,
Even with an external antenna you wont be able to get much range with the drone underwater. 2.4GHz is both the frequency that WiFi operates at and waters resonant frequency (microwaves use 2.4GHz and you might even notice your WiFi performance drop when its turned on).
Unfortunately getting an external antenna requires some fine SMD soldering - and an antenna connection at that (The connection should have as low of an impedance as possible (Without getting into it deep, impedance is like resistance but with capacitance and inducance tied in as well))
Here’s a link to someone thats got it working (its definitely doable with some practice!!)
