The short answer is: very. 🙂
So you picked up a Maltese travel brochure and saw these glorious photos of the Blue Lagoon in which the water is implausibly blue, a shade known by people who care about such details as ‘cyan’. And you weren’t born yesterday, so you conclude that the colour of the sea water is the result of a photo filter and the name of the lagoon is probably an advertising gimmick.
And you’re wrong.
The Blue Lagoon is so unmistakably and differently blue that you can spot it easily from the air:
And it remains that colour when you get closer:
If, after seeing the pictures above, you have now decided that you have to visit Malta and see the Blue Lagoon for yourself, a word of warning: avoid going in high season. It’s a very tiny place and even at the end of October it was crowded. It’s also rather short on beach facilities so…
… so the best way to visit is probably on one of the many day trips for tourists from places like Sliema. It means you’re taken there on a reasonably sized boat which remains anchored in the bay so that you can get back on board to use the bar and facilities during the day. You’ll be thankful for having a comfortable base to return to when you see the crowds.