Problem with the Cutwater 30 water and fuel tanks is that they are extremely long and have only the one sending unit mounted in the aft section of them so when the bow rises the level shows nearly full as I'm sure you've seen. Best solution for accuracy I've seen and thereby implement on mine was to add a second sending unit to the bow portion of the water tank.
With two resistive sending units, the brand KUS makes a dual sender junction unit which can be used to average two senders together to a single gauge, or split one sender and feed two gauges. Originally I bought and installed this unit so that I could connect the second output from the original sender to the Volvo EVC rudder/fuel/water harness cable so I could have water tank level displayed on the Volvo and Garmin systems as it is bridged to NMEA by the Volvo gateway. However after one season of use I realized how inaccurate the tank is because of the single sender, so I ditched the analog water gauge and reversed the KUS part so that I could install a forward sender and have high accuracy that is available on the helm displays along with alarms, and best of all it bridges through NMEA to my Victron Cerbo and is then available on my phone or computer anywhere in the world as long as the vessel has cell/Internet service, along with ALL of my other system data such as bilge pump running/counter, humidity/air sensors, battery state, shore power state, solar, etc.
If you want to add a forward sender to the tank it's a bit of a pain but doable... Remove the cave cushions/bedding and dri-deck (if you have it) and remove all three of the centerline vertical panels, then remove the starboard/wood blocks that the panels were screwed into. Disconnect all the hoses from the pickups at the back of the tank and slide the tank over into the cave where you'll have enough headroom to drill a hole in the tank and mount the second sender.
I suspect you could access the forward part of the fuel tank from under where the water tank was, however I didn't feel like it was necessary as the fuel flow data does a decent enough job of estimating fuel used that more accurate tank data wasn't a priority for me.
