Using SDKMAN with the Fish shell. It provide the sdk command and let you to use it transparently.
$ fisher unreliable-angels/sdkman
Before using commands which SDKMAN provides (e.g. kotlin), you have to run at least one sdk command.
As a workaround, running sdk help or something in your config.fish will solve this problem:
if type -q sdk
sdk help > /dev/null
end