The only way to reach Marstrandsön is by boat or ferry. The ferry only takes a few minutes though: by the time you get on the ferry, you might as well start walking off again, because you are already arrived at Marstrandsön ;-) The ferry “Lasse-Maja” crosses over to the island many times a day. On average it crosses over every 15 minutes, but at night time approx. every half hour. The ferry costs 15 SEK for a return ticket (approx. 1,60 EUR / 2,15 USD). You can buy the ticket at the booth right by the ferry, or else, if the booth is closed on board the ferry.
There are no cars allowed on Marstrandsön, so you better bring some comfortable shoes! That is if you want to explore a bit more of the island of course. The ferry arrives in the middle of the village, so exploring the village on foot is easy. The fortress on the other hand is a bit of a walk uphill, and requires a bit more effort. Besides a visit to the village and the fortress, I really can recommend a walk around the island: hence the pair of needed comfortable shoes.
To reach Marstrand we took a bus from Goteborg central station...there's one every hour... and after 45 minutes we reached the town.
Marstrand is splitted into two parts and one part is an island. To reach the island you have to cross with a barge and you have one going back and forth every time.