Correctly applied silicone of the right grade is a (semi) permanent solution.
Like any other submerged seal it requires regular maintenance and inspection, but if you get a silicone that's designed for continuous submersion in water then it will hold.
Known as "oil canning" this is a common problem with plastic kayaks caused by long term storage or tight loading straps on roof bars.
Luckily you have a minor case and since this isn't a performance boat, the effect is entirely cosmetic. However it can be restored with the application of heat and pressure.
The best heat source is a hot sunny day. ...