Scotland has over sixty inhabited islands, and nearly fifty of them have scheduled ferry links. Most ferries carry cars and vans, and the vast majority can - and should - be booked as far in advance as possible.
Caledonian MacBrayne (generally abbreviated to 'CalMac') has a virtual monopoly on services on the River Clyde and to the Hebrides, sailing to 21 islands altogether. They have two types of reduced-fare pass. If your clients are taking more than one ferry, it's worth suggesting the discounted Island Hopscotch tickets. If they're going to be taking a lot of ferries, they might be better off with an Island Rover, which allows 8 or 15 consecutive days' unlimited ferry travel. It does not, however, guarantee a place on any ferry, so booking ahead is recommended.
Car ferries to Orkney and Shetland from Aberdeen and from Scrabster near Thurso are currently run by NorthLink Ferries. Pentland Ferries run a car ferry from Gills Bay, near John o'Groats, to St. Margarets Hope, Orkney, while John o'Groats Ferries run a summer-only passenger ferry from John o'Groats to Burwick in Orkney. The various Orkney islands are linked to each other by services run by Orkney Ferries.
Shetland's inter-island ferries are run in conjunction with the local council, and information about routes and timetables can be found on the Shetland Islands Council website.
There are also numerous small operators round the Scottish coast that run day-excursion trips.