But most pirates weren't able to swim -- they stayed aboard the ship, given that the waters were either too cold to survive long, or that boats most very quickly compared to a swimming man, and it's easy to get lost in the waves. That is, there was no benefit to swimming. I would say it would make more sense for lizardmen to be amphibious [not because of their biological nature -- that makes no sense, as pretty much the only lizards that can swim are crocodile skinks and water dragons, and I find it unlikely that the lizardmen descended from either, but I digress] in that such races are generally assumed to come from marshes or swamps, where a large river might have many tributaries, and thus navigation by canoe would be viable and useful.
What I'm saying is, the only feasible way of having an amphibious unit as I see it is to get someone who can actually swim [unlikely, considering everone's amazing proclivity for drowning in TT] or to have someone who can bring a lightweight boat along the land with them. It would have to be a specialised class of lizardman, but I feel it would, overall, make more sense.