telephoto lens needed for Rebel XT

Currently I have a Zoom Wide Angle-Telephoto EF 24-85mm f/3.5-4.5 USM Autofocus Lens.

Looking to purchase a telephoto lens for my Canon Rebel XT. I will be using it for landscape shots and for marine life while out on a boat. My budget is around $450. Any recommendations would be greatly appreciated! :)

Thanks so much!

18-200 3.5 from sigma or canon should be great for you. They run just under 500 atm

Thank you so much. I believe I am going with the Canon 70-300mm f/4-5.6 EF IS USM, thinking the IS will help me with the shots from the boat.