I remember that you used to be able to do, at least in theory, Exeter - (First Southern National X53) - Weymouth - (Wilts & Dorset 187) - Poole - (Wilts & Dorset X33) - Southampton - (First Provincial 78/79) - Fareham - (First Provincial 1/5) - Portsmouth - (Stagecoach Coastline 700) - Brighton all on one Explorer ticket issued by certain operators. I have done Portsmouth - Exeter before on a First Provincial Explorer ticket, but not sure if it was possible to cover the whole area.
Now the connection between Ringwood and Southampton is a little dodgy I reckon.