Research Tails and Flippers used for Swimming

Looking at aquatic animal tails and flippers will help with creating the paddle like tail and flippers for Tylosaurus.

Sea Snake Tails 

Other Aquatic Animals

Royal Clown Knife Fish

Salt Water Crocodile

Zebra Shark

For an extinct reptile that swims the nearest living example that has flippers are sea turtles

Loggerhead Turtle

Leather back Turtle