The optimal arrangement of buoys is determined in terms of accuracy and availability depending on the number of available buoys and targeted water depth for an efficient operation. For example, at a targeted depth of 30 m in fresh water, a maximum horizontal position root-mean-square (RMS) error of less than 3 m can be achieved with a set of five buoys providing a service radius of up to 72 m.
https://doi.org/10.3390/rs12213636
The formal transfer of GNSS under water is not possible. The radio signals don’t reach very far underwater, they’re basically useless for any submarine below the surface. If it is work underwater GNSS transmission is not feasible. The radio waves don’t go very far underwater, thus any submarine below the surface is effectively blind. If you have to work underwater, make sure it isn’t too deep. At around 1 foot deep, you’ve already lost roughly 15dB of signal. In the open sky, you can lose roughly 25–30dB of usual signal strength and get a GPS lock.