There are many good venues, but we recommend Les Basetes near Calpe and it may also be worth exploring further afield - Granadella and La Barraca (Jávea) are very scenic locations. You can rent snorkelling equipment from Les Basetes for around 10 Euros a day.
Take along some stale bread to feed the fish and you will soon be surrounded by a good variety of species. You may also see Octopus which are quite inquisitive.
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https://www.abdet.com/water-sports/snorkelling#sigProId87b9e86ad6
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