travel time arlanda airport stockholm to helsinki ferry

Tallink Silja website gives pretty detailed public transport directions between Central Station and the terminal. Last bus to make the Helsinki boat leaves station at 15:35, thus you have 1 hr 20 min from landing to get your bags and hop on the Arlanda Express train to Central Station (every 15 minutes, 20 minute ride). Seems pretty tight to me. http://www.tallinksilja.com/en/shipsAndTerminals/terminals/stoVar/default.htm
Viking terminal is about a mile from Central, taxi might be the way to go.

If it were me, I'd probably play it safe and spend the first night in Stockholm just to minimize the risk of missing the boat if my flight were delayed. However, if your flight arrives on time, you should be able to make the boat at 17:00, but you won't have lots of time to spare. I'd suggest just taking a taxi from the airport as this will be much simpler. At the airport, taxis will be queued up by company. You should use either Taxi Stockholm, Taxi Kurir, or Taxi 020; taxis in Stockholm are not regulated, and these three companies are all legitimate and offer fair pricing. Taxis offer a fixed price from the airport. The price should be about 520 SEK. Make sure to ask what the price is before you get in the taxi. You can check the taxi company websites for the current price. The taxi ride from the airport will be between 45 minutes and an hour. The other option would be to take the Arlanda Express which takes 20 minutes to the Central Station. From here, you would need to transfer to either a taxi, bus, or underground (Tunnelbana) to get to the port. The websites should have details on how to get to the ports. Silja departs from Värtahamnen while Viking departs from Stadsgården. Even though the train is faster to the Central Station than a taxi, by the time you switch to the tunnelbana or bus and then walk to the port, you won't have saved any time over a taxi direct from the airport.