Fullbring I'd say is close enough - if you skip it you'd arguably miss very little as long as someone tells you "Ichigo got his powers back and these characters were involved, sort of." Fishman Island's rather important regardless of quality - if that's filler then so is Little Garden, Thriller Bark, and a number of other arcs I'm sure. The arc had been explicitly planned for a long time and was a necessary turning point in the series, plus the Poseidon reveal especially is *kind of* a big deal.
As for Gintama, I don't know if I'd really call anything that doesn't pertain to any kind of plot or Gintoki's past or something of that ilk to be "filler." That's just the type of series it is. Filler implies that you can skip it or that it's just pointless in some way (regardless of quality), and in the case of Gintama you definitely shouldn't because it's primarily a comedy series at the end of the day. It'd be like calling most episodes of a sitcom filler, bar the ones with a sizable amount of legitimate drama and/or character progression/regression.