The first suitable ferry from Dartmouth is the lower vehicle ferry at 07:10, Mon-Sat, which suggests an earliest practical rail departure around 07:30 which could plausibly get to Exeter before 09:00. The railway company passenger ferry starts at 07:30. The upper vehicle ferry starts a little earlier at 06:30, and can also be used as a foot passenger, but is a long walk from the station at the Kingswear side. Service finishes around 23:00 on all routes. Run by different operators, fares for a single passenger journey are the same on each crossing I think, but returns and multi-trip fares aren't inter-available, and inclusive railway tickets can't be used on either of the local authority run car ferries.
It's an unusual line for the 'heritage' sector in having centrally controlled mainline style TCB signalling with colour light signals, full track circuiting and electric point machines, so only needs one signaller on duty at Britannia Crossing in order to run trains. The NR-owned Sands Lane crossing at Paignton is operated by traincrew and has dual control units, one for mainline crews going to and from Goodrington sidings and one for the steam railway staff. 'Passive provision' for a fully signalled connection between the two networks was made in the 1990s relay interlocking and control panel, but has never been followed through on the Network Rail side, hence the NR end of the crossover points still need to be clipped and padlocked and the driver verbally instructed for each movement, limited to infrequent excursions currently. Regular daily movements would certainly require something more secure than that.