I dived Isla del Cano with Oceans Unlimited a couple years ago. They are located in Quepos.
Looking at your options - it appear you have three within reasonable proximity: Southern Expeditions, Oceans Unlimited, and Mad About Diving.
Southern Expeditions is closer to get to, but they launch their boat from Sierpe, which is quite inland, and takes 1.5 hours in their van to get there from their location in Domnical. Then they take you on a 45-minute boat ride down a river before they even hit the ocean. They don't tell you how long it takes to get to the island from that point, but it appears to be about 1/4 the distance that it takes Oceans Unlimited, in Quepos. So you need to get yourself to Dominical, which is about halfway between Ojochal and Quepos (going in the OPPOSITE direction from Sierpe) by 6:00 am...or, you can drive yourself to Sierpe (on windy, poor back roads) and get there by 7:30 am to meet up with the boat. At that point you'll have breakfast, board the boat at 8:00 am for the 45 min. ride downriver, then however long it takes to get to Isla del Cano (I would estimate 30 minutes but I could be wrong on this).
Oceans Unlimited departs from their location near the dock in Quepos at 7:30 - no long van ride, no boat ride down the river, just board their boat and go. Their website says it takes about 1.5 hours to get to the island, but it took us a little over 2 hours - although we left later, so the waves had kicked up more than they do earlier in the morning. Quepos is a straight shot up Hwy 34 from Ochotal, but you should probably allow at least an hour to get there - but it's all highway. So you leave Ochotal at 6:15, arrive in Quepos before 7:30, and get right on the boat for the trip to Isla del Cano.
Mad About Diving - their website appears under construction, so I don't know anything about them!
Having done Isla del Cano with Oceans Unlimited, here's what we can tell you: their boat was not particularly comfortable (hard bench seats) and the ride to the island was pretty brutal (2 hours of solid pounding through big waves, no protection from the huge sheets of spray from the bow hitting you square in the face), but that experience was condition-dependent - it might not be so bad if the conditions were better. We were unhappy with them for booking our elderly friends to come along with us to snorkel while we dove, which they couldn't do after having their kidneys and backs pounded in the boat for 2 hours! They have since put a disclaimer on their website that reads "This is an average journey time, can be longer depending on wind direction and conditions. We make every effort for the comfort of our passengers, but we would not recommend this trip for the elderly or very young (minimum age is 5 years old), please contact us for more information." And they reimbursed us for their trip, so kudos to them for that.
The boat also has no head, so we had to stop mid-trip for the guys to pee off the back (not very accommodating for the gals!
).
But once we got to the island, the diving was FANTASTIC! And we LOVED our DM. We live in SoCal where divers are expected to be self-sufficient and dive with their buddies, and DMs don't go with you as guides, so we usually dislike diving in destinations where the DMs expect you to dive in a group and stay with them like little children, and make us all end the dive when the first person breathes out their tank. But this DM handled things differently - he did expect us to stay together, but he made sure we were all buddied up. And as people breathed out their tanks, he'd send them to the line to do their safety stops, and allow the rest of us to continue. In fact, when my husband breathed out his tank before I did, the DM actually sent him to the line with another couple who were also finished, and continued the dive with just ME! It was a really unusual DM-led dive experience, and one I really appreciated. Most DM-led dives that we've been on around the world end sooner than we'd like due to noobie hoovers in the group.
And the diving was great! HUGE schools of fish, reef sharks, lots of big eels, and rays. In fact we saw a couple of devil rays get snapped at by a shark and take off like shots - that was pretty cool! The vis was really good, better than I was expecting. It's not warm-water Caribbean diving and brilliant corals, but there is a LOT of life under there to see. Very worth it.
Don't know if this missive helped any, but good luck! We're currently planning our THIRD trip to Costa Rica, this time to Playas del Coco, which we haven't been to yet.
