What do you consider a 'heavy lens". Unfortunately all the quality telephoto lenses are heavy, with the long ones being 'extremely heavy'. Long range with wide aperture just requires a lot of glass, there's no two ways about it.
A teleconverter is better than nothing, but it won't replace good long glass. They're also not cheap and cut your largest aperture in half. It could be sunny, but it's Antarctica and I'd want to be prepared for cloudy and overcast (i.e. large aperture). Honestly at that point I'd just get a cheaper third party telephoto with variable aperture. Sigma makes a 70-300 that are super cheap. If you're ok spending a little bit (which is peanuts compared to a trip to Antarctica) I think they make a 150-500 which isn't small, but not terrible heavy (~3 lbs) and that's quite a range on a crop sensor.
What kind of things are you hoping to photograph? Lenses I'd seriously consider even if it meant selling them after the trip are the Canon 70-300 L IS or 300 f4 L IS or maybe the 400 f 5.6 L. Using a TC with the 300 might work but AF may be sluggish, & I wouldn't try it on either of the others unless you have very good manual focus skills. The 300 & 400 are relatively common on used equipment sites & if bought carefully could be sold for the same price after the trip.
Going to Antarctica but not getting off the ship. Any suggestions for a telephoto lens. I have Rebel Xsi getting T5i. How about a teleconverter? I can't use a heavy lens.
It would depend on what you'd like to photograph.
The first being more "flexible" in focal lengths but not so sharp nor "fast" as the second. Anyway it's a GREAT lens. If you want high quality image, you should consider using high quality lenses.
With 5D Mark 3, the 70-200mm 2.8L II + 2x EF Extender III works really well regarding AF, but we didn't test it on the T5i. The AF performance may be very different.
I was thinking about going there too, but getting off the ship for sure 🙂 In my case I'd take wide angle and telephoto lenses for different subjects and topics (great landscapes, cientific bases, distant objects, animals, etc).
Take care of the cold weather for you and your gear. Don't bring the gear into a warm room suddenly from cold outdoors without putting them into a zip lock bag or covering with a towel to let the gear get the indoor temperature gradually. Otherwise your gear will get moisture condensation and it may get damaged easily, especially if the camera+lnes is not weather sealed. Some other people just leave the gear inside the bag/backpack for a while until it gets warm.
Keep the spare batteries warm in your pockets, and take spare caps too, you never know when you'll need some.