Perhaps a Dytiscidae. Better photos are needed for identification.

This is a species of Hydrophilidae. I'm not familiar with the Tunesian species, so I cannot put a more precise name on this. There is literature you could use, or ask a person working on this group. But it is well posible that the specimen cannot be identified to species level based on these photographs.

Cheers,

Kevin

In North Africa and the Mediterranean regions, common genera include Hydrophilus, which are quite large, and Enochrus, which are medium-sized.

Your beetle appears to be on the smaller to medium side, which could suggest Enochrus or a closely related genus.

Many, many thanks, Dr. Peter