Well depends where you wanna go - there´s a boat from Gothenburg to Kiel... Or to Denmark?!
Try having a look at viamichelin.com - or just do a search on Google! You will find many a useful informations on Sweden + it´s geography! But I do believe it will take waaaaay too long sailing from the UK to Gothenburg!
Best bet I can give you is as mentioned to do a flight to Gothenburg and catch a train, both Ryan Air (not good for paintball though?) and SAS flies there on a daily basis - you can go to Copenhagen as well and take a train from there straight to Varberg, BUT that´s about 3 1/3 hours instead of only about max. 1...