You don't have to be in the subtropical parts of the US to find plants. I've collected Ceratophyllum, Elodea, Riccia, Ricciocarpus, Chara, Nitella, various aquatic mosses and macroalgae, Echinodorus, Sagittaria, Ludwigia, Azolla, Lemna, Spirodela, Wolffia, Vallisneria, Cabomba, Polygonum...