Read the phrase carefully and identify its type of Samas (Compound):
"जल में समाधि $\to$ जलसमाधि"

In the compound breakdown "जल में समाधि" ( जलसमाधि), the locative case marker/postposition 'में' (in) is omitted when combining the words into a single compound. When a compound retains the meaning of the final word and involves the deletion of a case marker, it is a Tatpurush Samas (specifically, Adhikaran Tatpurush).