Theertha (Theertha) literally refers to water. In Hindu sacred literature, it is referred to as the physical holy water body associated with a temple or deity.
The word 'theertham as in 'theertha yatra' has its roots in Tamil. Tamil prospective provides a much better and acceptable analogy.
The 'theertham' came from a Tamil word 'theervu' which stands for 'solution' (for any kind of problem faced by humans). It is from this word we have words like theermanam, thiruppam came in. The word 'Thirthankara' used in Jain faith also has its roots in the same Tamil word.
Thirthankara actually means 'Theervu + Thanda + Karangal" where Theervu is solution, Thanda is providing and Karangal is hands.
Therefore to refer to people who used to provide solution for day to day problems (usually saints sitting in temples provide such solutions) as a result they were called Theervu Thanda Karangal which over a period of time became 'Thirthangarar" or Thirthankar.