most dive ops use the "panga" a modified Mexican fishing boat, or something similar. Generally known as "6 packs" these boats have side ladders and can be a challenge at times to get up if the water is choppy. March is a little early for us, but I would imagine it is not much different than April, so seas should be not too choppy unless a late Norte is coming in.
the dive ops I've dove with over the past 20 years usually have you take your gear off and hand it to the captain before exiting. weight belt first ( if you are still using one), BC/rig next, fins last ... then it is much easier to climb the ladder.
I have also dove down there after being in a car wreck with a bummed up knee, and was able to navigate the boat ladder, with a little help.
I have several friends I regularly dive with in Coz that are in their late 60's and into their 70's, they all seem to have no problem getting on the boats.
just take your time, no one will rush you, the DM and Captain will do everything in their power to help you if you need it.
... enjoy your trip! ...
p.s. trip report's are nearly mandatory to be on the Coz forum ...