No - I've been there too.  This is much farther away.  Not a day trip.

This is what some friends of mine on a boat like ours wrote about it

Latitude: 05-33.097N; Longitude: 087-02.528W
June 24, 2001
Chatham Bay, Isla de Cocos, Costa Rica

We allowed ourselves to go very slowly the last day in order to
approach Cocos in the morning light as opposed to midnight. This
prudence rewarded us with a spectacular arrival. Around 4am the last
bit of moon rose with a bright planet alongside, and dolphins
paralleled the boat making comet-like vapor trails through the
bioluminescent plankton in the dark water. The island was black and
mystical, rising steeply to a cloud around the summit, and as dawn
gradually broke it became greener and greener until it was a color so
bright we couldn't believe it. Dodging the seasons back and forth
across the equator, we'd completely forgotten what a dense tropical
green could look like, although in truth I don't think we've ever seen
an island this lush anywhere! Waterfalls actually burst from the sides
and tumble directly into the sea! (Opening scenes of Jurassic Park
were filmed here!) As we rounded the point into the Chatham Bay
anchorage, hundreds of frigate birds circled the boat along with
dozens of boobies, to the point camera work on the bow could be
considered risky business! Ahead in the bay was just one boat..  We
picked up a mooring, had a bacon and egg feast and a short snorkel
just to remind ourselves what warm water was, and had just dozed off
in the cockpit when the park rangers showed up to do business. Before
we knew it we were all arranged to do a dive that afternoon.

A little background. Cocos, plus a dozen or so satellite rocks and
islets, is one of the premier diving destinations of the world.
Belonging to Costa Rica, it is entirely a Marine Park and is totally
uninhabited but for some park rangers and volunteers who reside in one
of two stations, a small one here in Chatham Bay, the other larger
base half-way around the north side of the island. There are no
facilities ashore for tourists, so all must come by sea. The bulk of
visitors to Cocos are scuba divers coming on one of two top-notch
liveaboard dive boats, the 115' Sea Hunter and the Okeanos Aggressor..
The other vessels that trickle in are either cruising boats like
ourselves, or fishermen, who, though they can't fish in the park
waters, are permitted to seek shelter here. The fees for cruisers to
be here are a bit steep, although not so steep as rumor had it. For us
it costs $15/day for the boat plus $15pp/day for us, which is a total
of $45 a day. About like being in a marina. For this you have a nice
mooring (yes, we checked all its attachment points!), access to fresh
water (they have a pipe rigged in a waterfall that carries fresh water
out to an offshore buoy!) and an onshore laundry tub and showers, none
of which we need on TII but which are welcome amenities for many
cruisers.

Diving requires having all your own gear, including dive compressor.
Fortunately, since ours is still down from the failed hose in
Galapagos, the Sea Hunter was here and the very friendly captain
filled all our tanks for us. Then, as he was leaving on Monday, he
went one step further and lent us a fill hose for our compressor. The
other hitches with diving here are we must be accompanied by a park
ranger and we have to get there in our own dinghies. This puts us at
the mercy of the rangers' schedules and restricts our range to how far
we can practically get by rubber duck. However for $4/day it's a good
deal as you get a guide and, as all the dives are drift dives, a chase
boat that tows the dinghy along after you.

The diving had been great. Similar to the Galapagos, the waters are
thick with fish, with 3-5' foot white-tip sharks as common as
trumpetfish in the Virgins and lobsters carpeting the rock faces
(which may well be a main reason we have to be shepherded by
rangers!). Unlike the Galapagos the water is warm enough for skins
only and the visibility is a clear 70', but there is still very little
coral, which we have since learned is thanks to the disastrous El Nino
of 1987. On one dive - Roca Sucia - we had dolphins leaping around the
dinghy topside and several schools of hammerheads swirling around the
underwater formations. Lot of marble rays and turtles too. We've seen
four huge Manta rays, but every one of them was from topside. It seems
they like to glide just inches below the surface with their wing tips
curling into the air! I did managed to get in with one briefly on
snorkle and we hope we got a photo of another at least 12' wide! The
downside of this paradise is the amount of rain they receive -- 280" a
year. There's a reason it's green! Given that, it's been relatively
nice weather during the time we've been here, with only one full day
of rain and but two days of nasty roll! The rangers say the best time
of year is November through January.

You can read the rest here:
http://www.thetwocaptains.com/logbook/fiftyone_sixty/volume51.html
grandma Rosalie