Scuba doesn't require certification, either, but most businesses require you to be
certified before they will rent gear to you, take you diving, or fill your tanks.
Since snuba *is* scuba diving why would you choose it just because it doesn't require
certification?

"Snuba" is a trademark for a company that markets a particular variation on surface
supplied air. They also have a patent, which is probably a good example of a patent
that should never have been issued, due to prior art and their particular application
being obvious. The tank floats in a small raft (that's what the patent is about), and
it's usually done from the beach. You don't need a compressor or a boat. Snuba limits
you to 20 feet, because that's how long the hose is, but commercial divers using
surface supplied air go much deeper. You can also get small compressors that float on
the surface and usually have a 40' hose. Snuba claims to have an excellent safety
record, but from a physical or physiological perspective it's no different than
regular scuba diving in most respects. Because your depth is limited it's extremely
unlikely that you could get bent, but if you're at all careful with regular scuba
it's still very unlikely. What can still happen with Snuba are the more serious
injuries that result if you ascend without exhaling. Because of the pressure
difference between the surface and 20 feet down any air that comes out of the
regulator at 20 feet will expand about 65% on its way to the surface. If you exhale
at 20 feet the bubbles will get much bigger as they ascend. If you ascend without
exhaling the air will still do it's best to expand by 65% with your lungs offering
some resistance. If your lungs are much more than half full that will almost
certainly ruin your day, since the strength of your lungs is no match for the power
of the expanding air. When the air expands it can rupture your alveoli (the
individual air sacs in your lungs, if you didn't know), and/or enter your arteries as
an AGE, or arterial gas embolism. If you don't know what an embolism can do for you,
think blood clot in the brain and you'll be on the right track. In theory, it's
possible to damage your lungs by ascending as little as 4 feet, but it's not very
easy in the real world.

None of this should scare you, since all you have to do to avoid such unpleasantness
is exhale as you ascend, and those injuries are fairly uncommon. The first rule of
scuba is to never hold your breath. You should always be either inhaling or exhaling.
Of course if you jump in the water with no training and something causes you to
panic, bolting to the surface may seem like a good idea.

Now for some really good news. You can very easily go diving on your vacation without
becomeing certified first, by taking a "resort course". Some here will tell you that
resort courses are terribly dangerous because the training is very abbreviated
compared to a real class, and others of us will tell you it's a good, and inexpensive
way to decide if you want to continue with a full course. For about $80 to $100
you'll have perhaps an hour of class time that explains things like expanding air,
and safety considerations, then you'll spend some time practicing basic skills such
as clearing water from your mask and recovering the regulator in case you spit it out
of your mouth. Once you're done with that you'll go on a dive that is supposed to be
limited to 40 feet for about half an hour. Compare that to Snuba, where the entire
process, including classroom, practice, fitting you with gear, and the actual dive,
lasts about an hour and probably isn't much cheaper (they've got similar costs to a
diving operator, plus paying fees to Snuba). Of course if you're already confident
that you'd like to really start diving you can take a class before your trip. You can
do the entire class, including he checkout dives at home, or you can do the classroom
and pool work at home and do the checkout dives when you get to the tropics. If
you're already certified 2 dives on a boat trip will cost you about the same as one
resort course dive or a snuba dive.

Visibility changes for a lot of reasons. Winter weather is usually rougher, so
there's a better chance that sand and other sediments will be stirred up in the
water, so that's one seasonal change. Cooler water allows more oxgen to be dissolved
in the water amd that can result in more life in general, and more plankton in
particular. Currents can change, and general weather patterns will have seasonal and
smaller variations. One of the reasons that the Cayman Islands are noted for good
visibility is that they are small, dry islands with no rivers carrying sediment into
the sea. Tobago is just the opposite, with large amounts of sediment carried by the
Orinoco River. One upside of bad viz is that it may be caused by plankton. Since
plankton is a major food source, lots of plankton may mean lots of critters.
Depending on where you are those critters can range from small filter feeders such as
sponges and tunicates, up to manta rays (soemthing Tobago is noted for), whale sharks
and real whales. However much stuff is in the water to reduce the viz, lots of bright
sunshine usually makes things nicer.

I didn't rule out using a boat. I suspect I'll confine myself to
snorkeling for now.