After you get your own cruiser go near the island that is shaped like a donut (I forgot what it's called) and start trolling. Select the white lure and wait until the rod bends torward the ocean. Follow the instructions on the screen to reel in the fish and at about 40 feet you will see the squid! Reel it in and it will jump right out of the water and onto your boat! Now you have a squid!
When you go to Hotel Fishing Resort, wait until a message where you can register for a trolling tour in the ocean appears on the bulletin board next to the fishing center. Register and when you set your tackle, select the WHITE lure. When the lure gets hooked you should see a huge gray fish flipping in the water. Reel it in and if this is your first time, a message appears saying: "Caught Xiphactinus for the first time!". That's how you catch the Xiphactinus Fish.
For the Aligator Gar go to Panas Jungle and rent a raft. Ride to the Mangrove Pool (if you don't know where that is, open the map by pressing the - button.) and switch to a rod, reel, and bait for XL Fish. Do what you normaly do when catching a fish. You will get a lot of Peacock Bass and Redtail Catfish but eventualy you'll get a gar.
The Xiphactinus appears in the seawater section of the aquarium in the third temprate zone tank. The Gar appears in the freshwater section in the first tropical zone tank.