Antenna Design for Underwater Applications by Oluyomi Aboderin
It is a long and often technically complex paper but he recommends Chapter 2.5 for a good historical review and the section starting on page 111 gives details of antenna mounted on the outside of the hull or WTC.
The diagram on Page 111 shows the general idea - a dipole mounted on the outside and insulated from water by a thin covering.
You may wonder what the advantages may be? Well space saving is one. Also the antenna can be much shorter (i think he said 1/10 of the length in air.) This would have particular advantages for 27MHz.
We need to do some practical experiments to see how these work out - not too difficult to build, I think.
Rob recommends dipoles.