Final answer:
Pi alerts the ship of his presence by waving a bath-towel tied to a stick and relies on the lighthouse keeper to see them and notify rescue services.
Step-by-step explanation:
To alert the ship of his presence, Pi undertakes two actions according to the provided context. First, he makes use of a bath-towel to create a visible signal. This bath-towel is tied to a stick, and the captain waves it in an effort to attract attention. Second, the captain hopes that the lighthouse keeper will notice them through a glass (presumably a telescope or binoculars) and notify the life-saving services. These actions demonstrate the ingenuity and resourcefulness often necessary for survival in desperate situations at sea.