Here's why:
* Imperative sentences give a command, request, or instruction. They typically start with a verb (in this case, "watch").
* Interrogative sentences ask a question. They typically start with a question word (like "who," "what," "where," "when," "why," or "how") or an auxiliary verb (like "do," "does," "did," "is," "are," "was," or "were").
Since "Please watch it" is a request, it is an imperative sentence.