Yep there's no cap on effective skill level obtainable to my knowledge. In addition to the aforementioned larger bar size, if you get up to 15 (say via that combined with Seafoam Pudding) you can cast 1 tile further out, which doesn't really have much impact since you can already reach the highest water zone at 10, but could theoretically come into play in some areas, especially for hard to reach bubbles.
Plus, your fishing skill impacts fish quality, so if you have say a fish which for whatever reason must be caught in a suboptimal fishing zone (such as the angler, or the night market fish, as examples) then having higher fishing skill can make them more reliably gold quality (iridium quality with the quality bobber or a perfect catch). Finally, having a higher fishing skill decreases energy consumption. If you get up to 16 fishing (the highest I know how to get) then you're only using 6.4 energy per cast instead of 7 at level 10 fishing.
All told, it's definitely one of the best options for the fishing rod. Preserving also seems like a great option, depending on what kind of bait and tackle you like to use.