Could a whale cause this?

[Naughty Cat]

We put out from Imperia (Liguria, Italy) a few weeks ago to head up the coast with an ok breeze, and a frustrating current from the north. A few miles offshore the depth sounder showed we were over the shelf at 200m+. The charts, as you would expect showed no rocks, obstacles etc..

Then the depth gauge showed 6.5m. Then blank, then 5.8m, blank, 6.5m etc and so on between 5.8 and 6.5m for 20 minutes or so. Eventually we tacked and it did the same at around 10m for about 5 minutes and then went blank again. There were no further anomalous readings that trip. I checked the depth sounder that night and it was clean.

Imperia is well known for whale sightings. We did not see any but it got us wondering whether we had something under us. Anybody else experienced something similar. Are we just wishful thinking?