SeaLand Tankwatch I...BUT--
I've never seen any point in having a gauge that only lets you know you should have pumped out before you left the marina...and unless you regularly inspect visually to how full the tank is getting, that's what usually happens if you have a gauge that only lets you know when the tank is full. If you have to make regular visual inspections anyway, why bother to spend the $50--which is about the price of the TW I? The AcuGage someone else mentioned is the best on the market. Nothing goes inside the tank--so there's no sender to clog and fail. Instead, wires in adhesive-backed foil strips go on opposing sides of the outside of the tank...they give you an infinite reading of the contents level by measuring the resistance between the wires. It only draws miliamps, but if even that's more than you want all the time, you can turn it on when you want to check the level in the tank. And, you can put up to 4 tanks on the same read-out panel. Cost, especially compared to the "multi-level" Sealand guages, is very inexpensive--only a little more than twice what you'd pay for something I consider barely one step away from useless. The mfr is Snake River Electronics...You can check out all the specs on their website athttp://www.snake-river.orgAnd you can buy it from the online store here.