They were going out to meet the ships of Octavius, which were lying at anchor not very far from them.

Antony supposed that his vessels were going to attack those of the enemy, and he looked to see what exploits they would perform.

They advanced toward Octavius's ships, and when they met them, Antony observed, to his utter amazement, that, instead of the furious combat that he had expected to see, the ships only exchanged friendly salutations, by the use of the customary naval signals; and then his ships, passing quietly round, took their positions in the lines of the other fleet.

The two fleets had thus become merged and mingled into one.
Antony immediately decided that this was Cleopatra's treason.

She had made peace with Octavius, he thought, and surrendered the fleet to him as one of the conditions of it.