I think it's because it's a photo that was edited, and the boat is the part that was drawn. Zoom in on the boat and you will see the difference in the quality.
The inconsistency you talk about is probably a result of low quality photo being used as a base.
canaridante t1_j047rkz wrote
Reply to comment by Noroark in Endless sea, me, digital, 2019 by Kirill_vfx
I think it's because it's a photo that was edited, and the boat is the part that was drawn. Zoom in on the boat and you will see the difference in the quality.
The inconsistency you talk about is probably a result of low quality photo being used as a base.