My trip to Mexico was the second time I regretted not bringing my own snorkel gear with me. My fins are old, heavy, all-rubber ones, not really comfortable for tropical diving/snorkeling ( since tropical divers do not always need wetsuit booties, many use long, full-foot light pocket fins while diving in tropical water ). Also, I wanted to travel light, and knew that most dive schools provided all the gear for their trips.
But even if you're not a diver, in places like Mexico ( Hawaii or Florida's Keys ) there are many great places where you can go snorkeling.
Of course, you can always rent a snorkeling set ( mask, snorkel and fins ), or even buy a brand new one at a local WallMart. But having you own snorkel gear means you’ll have one less thing to do when you get to your destination. In other words, you’ll be at the beach sooner instead of searching for the rental shop.
I think that if you're a frequent visitor/lover of sunny tropical places ( and love snorkeling/diving/swimming ), investing in a quality snorkel gear set is a must.
As I've mentioned above, full-foot pocket fins are lighter, and sometimes the whole snorkel set will weight about 4 lbs. A set usually comes in a convenient carrying and storage bag, that will easily fit in any suit case.
Also, before I heard people recommend bringing your own snorkel gear because of the hygiene aspect ( Mexico doesn't have the same health & safety rules that you're likely used to at home ). I don't know about you, but for me, there is just something about having your own snorkel in your mouth :) When it comes to masks, aside from the hygiene thing, pink eye can be very nasty.
And finally, the price of the snorkeling set is ( almost ) the same as the price of one (!) "snorkeling tour". So, buying and bringing your own gear will pay off even during a short tropical vacation.