British Nationalised Shipping (Clegg W & Stryng J, Augustus M Kelley, New York, 1968) provides a history of the railway-owned ships and services from 1948, with some detail of history before that date.
As mentioned previously, in 1948 the LMS operated TSS Tern, TSS Swift, TSS Cygnet (although she didn't last long), TSMV Teal, and TSMV Swan on Lake Windermere; the steamers Gondola and Lady of the Lake ran excursions on Coniston Water until 1939.
In Scotland LMS/LNER "Group Committee No 4" operated steamers on Loch Tay (until 1939); Loch Awe, using CSP Countess of Breadalbone (until 1951); and Loch Lomond, using paddlers PS Prince George, PS Prince Edward, and PS Princess May - Prince George was broken in 1922, and Princess May withdrawn in 1952, Maid of the Loch entered service in 1953, and Prince Edward was withdrawn in 1954.
On the Clyde the LNER operated ships from Craigendoran, whilst the LMS-owned Caledonian Steam Packet Co Ltd operated out of Ardrossan, Gourock, Largs, and Wemyss Bay. Over on the Forth the LNER operated a ferry service between Granton and Burntisland which was suspended in 1940 and then formally abandoned in 1947. But, I suppose thse would also be coastal, as opposed to, inland operations.