From Wikipedia's plot summary of Raiders of the Lost Ark:
(all images are taken from here)

Indiana Jones and Marion leave Cairo to escort the Ark to England on board a tramp steamer. The next morning, their boat is boarded by Belloq and the Nazis, who once again steal the Ark and kidnap Marion.

Indiana stows away on their U-boat and follows them to an isolated island in the Aegean Sea where Belloq plans to test the power of the Ark before presenting it to Hitler.

My Question:

• How exactly did Indy "stow away" on the submarine?