Radio communication under the sea is not an attractive option for experiment by the radio amateur as it requires the use of very low frequencies, large antenna systems and very high powers.
Fresh water lakes and rivers have much lower electrical conductivity than the sea and underwater transmission distances (or depths) up to 30 metres appear feasible using the [then-not-now] lowest frequency amateur band of 1.8 MHz. Even larger distances (or depths) could be achieved if a lower frequency band allocation were made available.
However, that was written in 1987 and from a cursory historical review of https://en.wikipedia.org/wiki/ISM_radio_band it seems that exactly around the late 1980s time that article was written, the still earlier-suggested lower frequencies within the ISM bands were popularized and confirmed by various national regulators and there are now commercially available ISM-band transceivers available, eg. https://hoperf.com/modules/rf_transceiver/index.html
You can use directional (high dBi / high gain) antennae at one or both ends of the link, however this adds substantial complexity and generally negatively affects the vessel hydrodynamics.
Let's say a sub delivering critical medicines to peoples in need is supposed to cross an ocean, it probably wants GPS at least sporadically (for inertial navigation error correction) and AIS (to avoid collisions with cargo ships in busy shipping channels) which means it's operating at least near-surface most of the time anyway which means a radio comms tower is likely the best option, and the question is moot.
OTOH, if a sub is launched from a research vessel, it's cheaper and easier to use a tether. If it's just for a hobbyist, then the commercially available ISM transponders are likely adequate. So as per the 1987 comment: it seems in-water long distance comm's is really the domain of the military due to their unique requirement for deep-water stealth and the corresponding need for unlicensed low radio frequencies. For the rest of us, use ISM-band, a towed surface relay (also stealthy anyway), a tether, or just surface for comm's.
Radio communication under the sea is not an attractive option for experiment by the radio amateur as it requires the use of very low frequencies, large antenna systems and very high powers.
Fresh water lakes and rivers have much lower electrical conductivity than the sea and underwater transmission distances (or depths) up to 30 metres appear feasible using the [then-not-now] lowest frequency amateur band of 1.8 MHz. Even larger distances (or depths) could be achieved if a lower frequency band allocation were made available.
However, that was written in 1987 and from a cursory historical review of https://en.wikipedia.org/wiki/ISM_radio_band it seems that exactly around the late 1980s time that article was written, the still earlier-suggested lower frequencies within the ISM bands were popularized and confirmed by various national regulators and there are now commercially available ISM-band transceivers available, eg. https://hoperf.com/modules/rf_transceiver/index.html
You can use directional (high dBi / high gain) antennae at one or both ends of the link, however this adds substantial complexity and generally negatively affects the vessel hydrodynamics.
Let's say a sub delivering critical medicines to peoples in need is supposed to cross an ocean, it probably wants GPS at least sporadically (for inertial navigation error correction) and AIS (to avoid collisions with cargo ships in busy shipping channels) which means it's operating at least near-surface most of the time anyway which means a radio comms tower is likely the best option, and the question is moot.
OTOH, if a sub is launched from a research vessel, it's cheaper and easier to use a tether. If it's just for a hobbyist, then the commercially available ISM transponders are likely adequate. So as per the 1987 comment: it seems in-water long distance comm's is really the domain of the military due to their unique requirement for deep-water stealth and the corresponding need for unlicensed low radio frequencies. For the rest of us, use ISM-band, a towed surface relay (also stealthy anyway), a tether, or just surface for comm's.