Koh Tao

Most travelers take the Lomprayah route: a bus or overnight train from Bangkok to Chumphon, then the catamaran (1.5–2 hours). Coming up the island chain, ferries run from Koh Samui (~1.5–2 hrs) and Koh Phangan (~1 hr) several times daily. Seas get rough from late October through December, and the crossing can be genuinely unpleasant or canceled in that window.

An SSI or PADI Open Water certification runs about ฿11,000–12,500 over three or four days, and many schools include or discount a room for the course. Fun dives run about ฿800–1,200 each, less in packs. Schools differ more in class size and boat comfort than price, so choose on reviews and instructor ratio rather than the last ฿500.

Yes, with adjusted expectations: it's a small, steep island whose best beaches (Sairee aside) are pocket bays. Snorkeling is the best in the Gulf straight off the sand, Koh Nang Yuan is a half-day classic, and the viewpoint hikes are excellent. If you want long swimming beaches and resort variety, I'd point you to Samui or Phangan instead.

Roughly March through September, with April–June the sweet spot for calm seas and 20 m+ visibility, and whale sharks possible in any month, most often reported between about February and September. The October–December monsoon brings swell, runoff, and canceled trips. Many schools still run courses, but it's the worst window to plan a dive trip around.