When Is the Best Time
The season starts middle of June until September. The highest chance for a sighting is also the peak season on the Hebrides in July and August. Anyway, no guarantee at all. I booked a four-day trip to increase the chance for snorkeling with one and I only saw a young one from the boat. One guy had booked a trip for the third time and our three meters tall shark was his first one. You need the right conditions and also a lot of luck!!
I'd recommend August now. This is also the month when the shark festival takes place at one weekend.
Where and Tips
Our guide Luke, a marine biologist, explained that young sharks disappear as soon as you get into the water. It only makes sense to snorkel with older and bigger ones. The sun brings plankton to the surface this attracts the basking sharks. With a high concentration of plankton, the sea appears green. If you book a several days trip for snorkeling with basking sharks don't have too high expectations, I would rather call it snorkeling with seals which were quite amazing and a great experience. Have a look to the place Cairns of Coll. I'll definitely try it again. Anyhow, the sea can be rough, and it gets quite difficult to snorkel and to follow the shark. The visibility is 5 to 7 meters in summer. Expect lots of jellyfish in the sea.
Facts about the Basking Sharks:
The sharks are feeding half a ton of plankton and have a liver at the size of a car. The basking sharks were hunted almost to extinction for their liver to get the valuable oil and the fins. It is the second largest fish in the world after the whale shark. Growing up to 12 meters but there is still a lot to discover about them. No one has ever seen one of this sharks mating or giving birth. It looks like that the Hebrides are a key area for feeding and mating. Luckily they are protected now.