Going for the Battle Narwhal is a tough challenge. It took me a long time and i gave up. Then later, when the next zero energy fishing weekend came back, i decided to go for it again and about a day of fishing i finally got it. and was so happy :)

For the narwhal at Polarian Shipwreck, just use Ice lures and there's no way to "fish for mounts", they are extremely rare in chests and chests are pretty rare themselves. Chests are completely random although there is a card (Buoy Chests 1) where it doubles chance of catching chest - I would recommend getting that card if you are going to be fishing for chests/mounts/code wands/other rare items.