HI Alberto
Concrete has been used in full sized vessels, some hulls were even made of concrete.
Whilst feasible it is rather permanent once poured so you will always have a heavy model to move.
Lead on the other hand can be removed so making transport much easier, plus there will be less chance of damaging fittings with a light weight boat.
As you already have a supply of lead I suggest you build some ballast boxes in the hull bottom and make a note of the lead required in each box to save faffing around at the lakeside adjusting trim 😀

Stick with lead, its denser and so heavier for the same volume.

Ian

Never heard of anyone using concrete in a model boat, but that doesn't mean no one has.
I would stick to your old fishing weights Alberto

Alan