The GSG database currently has 1128 entries for seacaves around Scottish coasts (including Orkney and Shetland)
Judging by the fact that only 141 of these have a length recorded (on this database), most of them appear to be unsurveyed.
I have been checking out seacaves on the E coast between Arbroath & Peterhead and on the adjacent N coast, and have done rough surveys of some of them.
My tally of confirmed seacaves on the E coast is sitting at 112 of which 9 are over 100 m total tunnel length and one of those is 270 m. Most, but certainly not all, are indicated on various versions of OS - with the 1st Ed 6" generally the most informative - doubtless reflecting a much greater interest in, and local knowledge of, the coastline by small boat fishermen in the nineteenth century.
The caves are in sandstone, sandstone conglomerate, granite and one in basalt.