Determining Sea Floor Bottom?

[World_Devastator]

In SH5 as we know it isn't like SH3/4 as you can actually see your boat bottom on the sea floor, my question is what are the veterans method to determine how far away from the actual sea floor you are? Last night while chatting we Defiance I was at a depth of 117m and my depth to keel said I had 14m to bottom but I bottomed out at 118m how do I not do this again? (I just want to iterate I am not going by visuals just depth of keel)

My guessing is just give myself 20m cushion...