Lived in jersey now for many years, been on the boat many times. The fast boat is around 3 1/2 hours to weymouth, stopping in guernsey for about 20 mins to unload. In calm seas and sunny weather its great, fast and easy to relax - but if you don't stomach the sea well then if its slightly choppy then you will feel it. Find a seat in the middle of the boat for less up and down motion if its not too crowded. We travelled to England last year at the boats wave height limit - 3 metres on the leg from jersey-guernsey and 1m cross channel and it wasn't nice, many people practically passed out. I stupidly thought being up on deck would make me feel better but all that did was get me soaked in cold, sea water coming up over the front plus the boat slamming into the waves made it dangerous to move about so I had to stay there until we reached guernsey. My car got damaged too as did 8 others as they get parked close.
But... jerseys a great island to see don't let that put you off
ps. the ferry can be expensive sometimes in my experience, may be worth looking into flights and car hire as an alternative.