SPT claim that they live to join up journeys. They're not as good as they claim to be! Getting the operators of all the wee bus firms to cooperate seems to be challenging, as does finding an effective zoning system; zonecard, for example is a bit of a farce. Calmac
and Scotrail do not cooperate on fares, only timetabling. This means that connecting services at Wemyss Bay will require a minimal wait, but you can't get a through fare
Indeed, there is no ticketing option combining the trains and ferries.
One option is the
daytripper ticket. This won't cover the ferry fare, but covers probably everything after that. However, I'm not sure if you could use it for just one person; you'd have to check with SPT unless someone on here knows what it's all about! (I've never used this). However, if you're going in a group, this would probably be the best option.
Another option is the
Roundabout Ticket. You would need to buy your ferry fare on top of this, as well as a ticket from Wemyss Bay to Bishopton. Furthermore, you are restricted to Greater Glasgow area and parts of Lanarkshire and Paisley. Check the map to see if it's suitable.
The finally from SPT is the SPT Zonecard. It's a very confusing ticket, but for £54.70, you can use all the services on this map (£19.50 if you're under 16). Again, the ferry is not included, and this is really more of a commuter ticket, so it's unlikely to be what you're looking for. But if you're going to spend a lot of time riding on trains and buses it could be a reasonable ticket for you.
Sadly, there's no easy option when it comes to trains in Glasgow; it's even more complicated when you throw the ferries into the equation. We don't (yet) have an Oyster-like system, which makes joining up journeys difficult and unappealing, despite what SPT are trying to do.
The final option is the
Freedom of Scotland Travelpass. This lets you get ferries all over Scotland with Calmac, and all trains with certain restrictions. Again, it's aimed at people touring Scotland rather than the daytrip you're probably looking for, and consequently it's a rather more pricey option. But if you were interested, you could pop up to Inverness or Aberdeen for the day with this ticket quite easily, and take the West Highland LIne another day.