I’ve never tried a same-day booking, but it may be an issue. The most highly-regarded shops will likely be booked. Boats may go out before shops open. Different shops pick up from different places, so if you find someone to dive with, you may need to get to the caleta, unless your shop picks up in town. You might be able to book something while you are there, but give them at least a day or two notice. You should also note that some shops will use the first day in the water to evaluate you to determine whether you have the skills to go to more challenging sites. For example, if you want to dive Devil’s Throat, it’s likely you will need to dive with the same shop at least twice.