There are three main ferry routes to the Isle of Wight: Portsmouth-Ryde, Lymington-Yarmouth and Southampton-Cowes. Depending on where you want to go on the Island, that may be your biggest decider
For all 3, you need to get to London Waterloo station to take the train. That isn't actually too bad though. From St Pancras, follow the signs to the underground (there are lifts and escalators available). Take the Victoria Line (light blue on maps) to Oxford Circus, and try to be towards the front of the train. Follow the signs to the adjacent platform for Bakerloo Line Southbound (brown on maps) to Waterloo. It's a step-free change, and can be done in under 30 seconds if you're towards the front of the train and level with the tunnel linking the two southbound platforms.
Then, take a train to where you want to take the ferry.
For the Southampton-Cowes route, there's a free shuttle bus from Southampton Station to the Red Funnel ferry terminal, and you can buy a through ticket from London to Cowes including the ferry. Red Funnel have page covering the public transport options.
For the Lymington-Yarmouth route, you take a fast train to Brockenhurst, do a step-free cross-platform change to the Lymington train, then the ferry goes from the pier terminal pretty much next to the station. There's info on the Wightlink page, click to expand their Public Transport section to see it all.
Otherwise, look at the Island Line promotions page for details of some of the offers and deals for the through train-ferry tickets. (Island Line is the name of the train operator on the island)