They won't spawn during the day unless it is a storm.
There is a check for spawn in sections and has a chance of spawning every 9 mins... so waiting in one small area won't help. Keep moving at medium speed.

Watch for them, they may spawn to the side or behind far enough to not aggro on you so you have to turn around for those.

Best chance is night during a storm. Must be in an ocean biome. <ocean on the mini map>