Ten projects reached a final investment decision in 2015, amounting to €13.3bn, compared to the €6.5bn in 2014.
Although 4GW of offshore capacity was built (BNEF figures), 3GW was installed and connected in Europe, mostly by Germany (2.3GW) but with the UK (556MW) and the Netherlands (180MW) also contributing.
Germany's high total was in part due to the backlog of connections left from 2014.
EWEA said the region's offshore capacity now stood at over 11GW.
"New capacity additions will be lower in 2016 than 2015 though should then rebound, and we can expect to have over 20GW offshore wind in Europe by 2020. The real question is what happens after 2020," EWEA CEO Giles Dickson said.
EWEA also said the average size turbine installed in 2015 had grown to 4.2MW from 3.7MW in 2014 and 3MW in 2010, indicating the rate of technological advancement in the offshore sector.