Can sea currents be dangerous for surfers? How common would this be? Should I ask locals about currents before surfing at a new spot?
My surfing/windsurfing knowledge is limited. But yes they can be dangerous and yes you should seek knowledge on currents before surfing anywhere.
There are currents that can force you against rocks, or into sea caves or under water of the dreaded rip tides.
Rip tides are nasty and can suck you out to sea. My mate was caught in one of these once and in a matter of seconds he was in seriously deep water.
Here's an animation of how rip tides work:
The tide builds up pressure behind a sand bank, eventually this bank breaks and the build up of water empties into deeper water, taking everything and everyone with it. To exit a rip tide you swim side ways to the beach, not against it. You won't win against the current.