Best swell direction: West, southwest
Best wind direction: East, southeast. Anything west of due south or north blows it out, although it can handle a bit of onshore winds better than Grindavik. Also, Northeast winds aren't as clean for some reason.
Best tide: Mid tides best. Will break at low tide, but can be a bit ugly. Will break at high tide, but can get a backwash on very high tides. Needs a decent swell to break at high tide. There is a peak right next to the cliffs on the north end that may break at high tide when the rest of the beach is flat.
How to get there from Reykjavik: Take Route 41 south towards Keflavik. As you get into the Keflavik/ Njardvik area (about 15 minutes outside Reykjavik) you will see the Naval Air Station on the left and a sign for Hafnir. Take the left towards Hafnir. Near the front gate to the base, you will see another left and sign for Hafnir (about a quarter mile from Rt. 41). Take the left to Hafnir. Follow the road straight. After 8 km you will go through the town of Hafnir. Continue straight out of Hafnir. The very next reef you see after Hafnir on the right is North Merkines. You will see a small farm area and house on the right and lose sight of the water briefly. Look for a street sign that says "Merkines". Immediately after the sign and farm, the water will come into view again. The small cove on the right is Evan's Reef. Continue past Evan's Reef for about 5 km. You will see a large peninsula on the right in the distance called "Hafnaberg". Past Hafnaberg and a small aquaculture farm on the right and a crevasse on the left, you will come to Sandvik.
There are two ways to reach Sandvik. The first turn-off dirt road to the right will lead to the north end of the beach. However, you will need to cross quite a bit of soft sand to reach the beach and it is a longer, rougher road.
If you pass the first turn-off, you will see a small bay and sand dunes on the right side. The very next dirt road on the right is the road to the south side of sandvik. As you make the turn, you will see a sign on the right that says Sandvik. Take the road about 1 km. It will open into a clearing; bear to the right. The road essentially ends here and you will need to walk or drive over the lava track. As you get to the top of the bluff, you will see the beach in front of you. It is possible to drive straight onto the beach with 4 wheel drive- DO NOT attempt to drive to the beach with 2 wheel drive. You WILL get stuck!
Notes: Sandvik is the only beachbreak that I am aware of on the Reykjanes Peninsula. Don't let the sand fool you, Sandvik can be very dangerous when the swell is sizable. There is usually some current and rips, the severity proportional to the size of the surf. Currently the middle of the beach is the best peak, with a nice right and left breaking on mid tides. At high tide, the norhtern most part of the beach may be better. The south side has a left and right that can be fun. Sandvik is tide sensitive and the peaks will move with the tide. Also, the peaks may switch as sand moves during storms.
Best at chest to head high, to slightly overhead. Over 2 feet overhead or so it will start to close out.