If you don't get answers here, ask on the Lonely Planet Thorntree Africa branch. Try to narrow your questions a bit.
I've flown between Dakar and Accra, between Bamako and Accra, between Bamako and Abidjan, plus a few routes I've long since forgotten. Airlines in West Africa come and go regularly, so you need current information. It's never been cheap, so if that's what you're hoping for.....
I've also traveled a variety of routes overland, but visa rules change almost as often as airline routes, and there've been a few civil insurrections and wars in the past decade too. Generally, it's always possible to get some sort of visa on arrival in Bamako--by air or land. Recently, visas on arrival in Burkina have been pathetically expensive, but I've heard that's changed again.
Were I you, I'd wait until I arrived in Dakar, find an agent and ask. These things are usually far easier to work out on the ground with someone who already knows the answers and options. Dakar, Bamako and Accra are all air transport hubs, so it shouldn't be too difficult (cost aside). Be aware your agent won't know the visa rules or costs for your passport.
Otherwise, do what everyone else does: take local transport (buses, taxis grandes, tro-tros) through Guinea or direct to Bamako, then via Burkina Faso to Ghana. It's a nice trip, in parts. Expect to spend about a week on the road, and to be worn out by the time you arrive--spending a couple of weeks or a month will make a fine journey.
Sorry, but asking about ferries is just silly.
Hope that's helpful.