This information is true regardless of which yacht you book your Galapagos trip on.  Galapagos is not like the Caribbean where the dive sites are numerous and close together.  Therefore you should not expect to make the 20 - 25 dives that you would expect to make on a Caribbean live-a-board trip. 
The number of dives which are done on any trip to Galapagos is dependent how much time you have to actually dive.  This is dependent  on a two of factors:
1.  The amount of time available on any given day.  The Galapagos Islands are located directly on the equator so the days in Galapagos are exactly 12 hours long every day all year long.  The islands are far apart and there are many of them.  There may be a two to three hour boat run between dive sites.   Darwin and Wolf are 14 to 20 hours from the other islands depending on seas, wind and current.   Most of this long run is done during the night but you still must leave the south by early to mid afternoon to get to Wolf by the morning.  When you are not up at Darwin/Wolf there are incredible land excursions to do.  Each land excursion typically lasts 2 hour land excursions plus on many days you can snorkeling with sea lions and penguins.
2  Safety issues - You must have surface interval between dives.  You must wait an hour or so after you eat before you dive.  You must wait a minimum of 24 hour hours before flying and most yachts will not start the last daylight dive of the day after to 3:00 PM.   This last rule is because there are very strong currents and the next nearest land is 4000 miles to the west.  A lost diver at night is in serious trouble.   If a diver is separated from the group (which is common) and the dive takes 1 hour, this give the crew 2 hours of daylight to find them. 
3.  Night diving is the exception rather than the rule.  Remember that this is open ocean diving with strong currents.  The main attraction is not reef but the pelagic large animals off the reef.  A lost diver in Galapagos is a serious problem especially at night.  Therefore night dives are done in coves and are not available on most nights.

To give you an idea of the number of dives, you can typically expect one or two dives plus one or two hikes plus snorkeling .  At Darwin/Wolf you have time to make between 2 - 4 dives the first and last day up there depending on what time you arrive and what time you have to depart for the south.  On days where you are there all day long you have time to make between 3 and 4.
On a 7 night Darwin Wolf trip you will typically make between 16 to 20  dives. 

What are the dive conditions like ?

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