Dipoles lengths underwater vs on land
The velocity of electromagnetic propagation in any given medium is given by V = c/(sqr[ErMr]) where Er is the relative permittivity of the material and Mr is the relative permeability of the material. In free space, these relative values are 1.

What you therefore need to know are these values (I have not looked around but I am sure they  may be found). It is quite likely that ocean salinity will have a bearing on these, and salinity of the oceans varies quite dramatically.

From that, Lamba' = V/f where Lambda' is the effective wavelength of the signal in your new medium.

This is an effective length, but the actual antenna can be much smaller by using techniques such as base loading where a large coil is used at the base of the antenna (hence the name).

PJS