I just visited the region. Via land, from Cancun to Holbox Island is a 2.5 hour drive (cost me about $300 round trip for a van for my party of 5). Then you need to a ferry to Holbox (about $15 pp round trip). To visit Cozumel, you can either fly directly into Cozumel, or fly into Cancun, then travel 45 minutes south to Playa Del Carmen. From Playa, it’s a short ferry from there to Cozumel. Cozumel and Holbox are in opposite directions from Cancun. Another option is to visit the whale sharks via Isla Mujeres. Much closer to Cancun therefore much easier to couple it with diving in Cozumel in the same trip.