The current model of BT Cruiser has a tow rating of the following according to the Gulf Stream website.
7500 lb. Hitch w/7-Way Connector & Brake Controller
I find a rating for your 2010 model of 5000lbs, however. If you don't have it, here is a link to a brochure of the 2010 series of BT units.
https://library.rvusa.com/brochure/2...ng_Cruiser.pdf
Typically, for safety reasons towing more than 80% of rating should be done with extreme caution, if at all.
The Amphicar weighs 2,324 lbs, and a trailer for it should be under 1500 lbs so you may be OK. Use your best conservative judgement and don't endanger others on the highway.
Chuck