The difference between a shirt they wear and a shirt they keep for painting.
Pick the design with the insider joke over the generic one. Anyone can buy a shirt with a fish on it; a shirt that references the specific absurdity of the hobby proves you have listened to them talk about it.
Editor's pickHobby-plus-holiday designs are the strongest gift in this category between November and January, because they solve two problems at once: what to get someone with one loud interest, and what to get them for Christmas specifically.

Outdoor shirts get worn over other layers, so err roomy. If you cannot get a measurement, a Large covers more adults than any other single size, and the unisex cut here already runs closer to a men's fit.