I have an even cheaper suggestion: Borrow a whisker pole or even the longer sort of extendable boat hook. Tape two extended boat hooks together if you don't even feel like leaning over the lines. Go to the point on your boat where the deepest part of the keel is; most of the time this will be slightly aft of the chainplates on a fin keeler. Tie a piece of crappy white bread or a cheap hamburger bun to the end of the hook, the spongier, the better.
Dip. Measure. Examine bun end.
Now you know what are sailing ancestors knew by swinging a lead line with a lump of tallow: the exact depth and the nature of the bottom.
This matters in a marginal mooring depth-wise because if it's a foot of loose silt over sand, I would worry somewhat less than if it was boulders or a rock shelf. I would still worry, mind you, but not that the first grounding would tear me a new seacock.