Hello,
I have not seen much precedence for top hats pre 1790s based on contemporary depictions, but you could probably get away with the odd top hat. A good resource I follow is this blog:
http://britishtars.blogspot.com/More commonly, they would wear simple knit caps, or else a tarred hat with a shorter crown (such as the standing oarsman is wearing in your image) . In fact., you could fairly easily cut down top hatted models to represent tarred hats. Heck, as tarred hats, they'd even look good if they're somewhat rough looking...
Hope this is helpful, and please do post images as you work on this!