It's not uncommon for divers to leave behind assorted gear items, especially 'consumables' like batteries, for the locals. Depending on where you're diving, older, but perfectly functional gear left behind is greatly appreciated. Places that are very remote, isolated, and economically depressed are the most appreciative of one's surplus gear/consumables. Also, those locations with high import taxes/tariffs are also appreciative of free left over gear/consumables. I'm not sure Mexico is THAT hard up but I guess that depends on the specifics. I'd be careful to still tip cash as your old gear may be pretty worthless in money terms so it could backfire on you if you don't take that into account.