When Is the Best Time
The best time for snorkeling is during summer on a sunny day and a calm sea. Regarding the weather, it's usually best from April to the beginning of July. From mid-July to August rain occurs more often. September is also a good month for snorkeling in the protected lagoon.
Coll is very busy in July and August. During the peak season, it is more difficult to get an accommodation. The shark festival takes place in August which is very popular.
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Where and Tips
This lagoon is located in the north of Coll. The rocks are surrounded by shallow clear water and a beautiful white shell sand beach. You need a boat or travel operator to get to this magic place. For me, it was a great experience to snorkel in 12 degrees "warm" water in a tidal channel. Some of us spent more than 2 hours in the sea. There is much to discover like nudibranchs, anemones, crabs, huge urchins, different kind of jellyfish (moon jellyfish on a pic) and the curious common and grey seals. The seals come very close as long as you don't move.
Coll is a unique island and perfect to slow down. It has two lovely B&B's, a Bunkhouse and a hotel with an outstanding chef in the restaurant in Arinagour. You get the best fresh and local seafood I have ever eaten. You can rent a bike at the post office from 9 a.m. to 1 p.m., but it is a bit pricey. The area around Coll has the second most sunny days of the UK. The beaches are stunning. You get to Coll by ferry from Oban which takes almost 3 hours. Close to Coll is Tiree also a beautiful island but not as quiet with a "bigger" airport.
In June it won't get completely dark for six weeks. That's the reason I missed the perfect dark sky without light pollution.
You may encounter otter in Arinagour harbor when you wander around early morning or late evening.