A good, easy and safe way is to go north east of Dawnstar (it's on your fast travel map at the beginning of game at north center of map). Go northeast until you find the ship wreck called Hela's Folly.
You'll find a camp here with precious stones and other bonuses. Just northeast of this camp is the beach. Wade in and swim under water to like 35 ft out north or so, then head back to the beach because you should now have 2
slaughter fish on ur ass :P
To improve blocking equip your shield, go into sneak mode, then rubber band your left trigger to be depressed... BOOYA happy 100 blocking-for armor either heavey or light.
Just don't use the shield... this is an automatic leveling trick I use, you just need to check on your controller around once every 15 minutes to ensure your controller doesn't go into power saving mode.
I just watch TV, and when I see the light go out - or don't hear it vibrate anymore -
I just hit the B button then back out to fight again. The trick with the shield is to point yourself at both fish at once until they both hit your shield.
Hope this helps :)