My fish finder took a dump

[Fiskadoro]

Originally Posted by bus kid

...it decides to become permanent water.

You're on the right track. It's a portmanteau combination of two words into one new word. In this case Temper to make stronger and more resilient through hardship, and Mature pertaining to or characteristic of full development. The Temperature of water at La Jolla is not permanent, but it's predictable even with it's ups and downs due to the test of time.