Your first problem is that Canggu is often not included in the free pickup service so you may need to factor this cost in.
everything is relative. There are services from Serangan (Blue water and www.giligetaway.com). These will be 2h30 at sea but short road transfer.
Both are very expensive. Only Bluewater has a padangbai service (for Ubud).
There are a few companies out of Padangbai which include free pickups/dropoffs in the "southern resorts"/Ubud but again Canggu often not included.
www.baliekajaya.com often touted at the tourist booths. Can often get returns for IDR600k. All their Gili services are out of Padangbai. They include free transport from "southern resorts" and Ubud. Niot sure about Canggu. Boats are ok but recent problems with poor seastate/cancelled services has highlighted their crapp customer service.
Newish Kid on the block is Patagonia Express. www.patagonia-xpress.com
They will include pickup from Canggu and dropoff in Ubud but they only do central point pickups/dropoffs. In Ubud that is the Coco Supermarket.
They are quite expensive at IDR600k but that will be a departure from Benoa to Gili T.
You may want to check that the fast boats are actually running; they were cancelled for about two weeks up until August 2nd. If you are only going to Gili T for two days, it may be best to reconsider, or look at flights instead.
once on Gili T you can snorkel with turtles in front of the Danima Resort. Can hire snorkeling stuff off the guys on the beach. North end of Gili T is a good snorkeling area. You can also get snorkeling trips around the local islands. 2-3hrs. Expect to pay/haggle for IDR200k.
I used Bluewater Express from Gili T to Serangan and was very happy with them. Wanted to book Gili getaway but they were already fully booked so I was happy to get tickets for Bluewater because although they are more expensive that was the fastest and most comfortable way of traveling back to Bali. Ekajaya can be also good if you don't mind to spend long hours on a boat or if you decide to go to Padangbai. In Gili T you will find thousands of people selling tickets to Padangbai and/or Serangan so just ask them about and they will tell you which companies go there.