There are different options to travel between the islands and on an island. If you are not going to stay on Mahé, you can take a propeller machine (Air Seychelles or Zilair) to some of the outer islands or to Praslin. All other islands are either accessed by helicopter (currently only by Zilair), or by boat. The ferry service between Mahé, Praslin and La Digue is called Cat Cocos (visit their website for schedules and other info). On your boat trip, you can enjoy the view from the top, which is especially great on the biggest of the three boats. Be sure to ask for upper deck tickets at purchase. By the time of our writing, the price is the same but you are not allowed to go up without the special ticket.
Talking about Ferries: it is wise to sit with a view out to the sea and looking at the horizon in order to not get seasick, especially when the sea gets rough. If you want to be on the safe side, there is a special medicine against seasickness. We use it every time we go on a boat - it doesn´t do any harm since we don´t do it regularly but it might save your day. When going with the Cat Cocos, there is a shuttle service from the airport to the inter-island jetty. Just look out for a bus with a rather small note in the window, or ask one of the travel agents in the arrival area. The transfer is free if you booked your tickets online and have your reservation with you. If you want to take a day trip from Mahé to Praslin or La Digue, or even vice versa - Cat Cocos in our opinion is your best choice. There is a snack restaurant at the parking area at the jetty, in case you are hungry or thirsty, but you can also buy something to drink or a snack on the boat later. It takes about 60min between Mahe and Praslin. To continue to La Digue you´ll either have a short stop, or you have to change to Cat Roses and after that expect another 20 minutes. Especially the boat ride from Praslin to La Digue is a really nice trip and waters are usually rather calm.