The Maretron sender uses ultrasound (I think) and attaches to NEMA 2000 network.
To be able to read the "top" of the tank you have to use their "focus tube" which raises the sender a few inches.
The focus tube also helps for more accurate readings while underway (planing).
The sender or tube attaches to the 5 bolt hole tank sender that's part of the plates on the tanks. Maretron has other install adapters.
Did our forward tank last fall, we haven't messed with calibrating it yet as it's pretty good out of the box, and was waiting to do them all at once.
Of course nothing is ever easy, for all the other tanks we have to add a 5 bolt hole because we used the 5 bolt hole tank sender ports for our fuel polisher set up.
And there's no access to the water tank plate so it's going in a plate we added for new pick ups many years ago.

After looking at other options, especially Oceanic Systems, Maretron looked to be easiest (also very available) and we already have some other Maretron stuff that's been good.