The fuel tank, water tank and holding tanks all have vent lines. The vent line is connected to a small through hull which is typically located fairly high above the water line. Higher than any water discharge, typically.
The vent through hulls are very small. Yours is clogged. There are typically three causes for clogs in the vent line.
1) There may be a low level loop that is trapping fluid. The vent line should have a loop that goes ABOVE the through hull. This ensures that any external water flows back out the through hull rather than into the tank. If there is a trap, air won't pass.
2) The vent throughhull has a screen which prevents bugs like muddaubbers or yellowjackets from building nests. This screen is aluminum and prone to corrosion over time. The screen or the through hull vent itself may likely need to be replaced.
3) bugs like muddaubbers have built a nest in the vent through hull.
Locate the vent line on the top of the water tank and follow it. Inspect the vent line for blockages. If no blockages, remove it from the through hull and then attempt to fill the tank. If it fills, it's the through hull itself.
This is a very common issue causing fill issues with water, and fuel and pump out issues with holding tanks.