polyurethane sealants are very effective, eg. sikaflex. avoid silicone sealants as they contain acetic acid which may damage the metal. in any case, leave the fittings loose so as to get a thick (1-2mm) layer of mastic then fully tighten once the mastic has set, thereby getting a good gasket rather than squeezing it all out while wet.
I always use a polysulphide sealant for underwater fittings. (LifeCaulk is the most readily available brand in chandlers, but you can get all sealants much cheaper from industrial suppliers). Some people use polyurethane (eg sika-flex) or even silicone. The former will give you a terrible job when you next want to change the fitting, and the latter I simply would not trust under water, especially if application conditions/curing are not ideal.
Polysulphide will also cure happily under water, so there is no problem replacing skin fittings between tides if necessary (assuming the rest of the job goes OK!!).
agree with this advice, but recommend you do not get any of the mastic on the underside of the nut or on the threads as this will make future reoval very very difficult.