Find the water line

Today I bought a 1980 Daysailer that has been trailersailed since new, never had bottom paint. It replaces a 1980 I had for 20 years, destroyed at my dock in Florida by Charley in 2004.
I keep it in the water so it needs bottom paint to retard barnacles.
But how can I find the waterline without putting it in the water and marking with tape? It is on the trailer now under the house. Once I identify thee line I can mask with tape and paint the bottom.
Any ideas? Thanks for the help.
I keep it in the water so it needs bottom paint to retard barnacles.
But how can I find the waterline without putting it in the water and marking with tape? It is on the trailer now under the house. Once I identify thee line I can mask with tape and paint the bottom.
Any ideas? Thanks for the help.