Jacaerys wasn't prepared for the battle in that ship at all. People told him, that the sea is a dangerous player to fight, and in addition bad day, you can die there.

But even with all the protests, he was a good heir. He needs to protect and defend the right of his mother to the throne. So he went for it

But he didn't expect to end in this situation, almost dying in the middle of the ocean, with only a piece of wood to endure his body covered with wounds and arrows... he needs a blessing right now, a saviour because he needs to claim the Iron Throne for her, for his mother.

Before he reaches the unconsciousness, he sees a little ship. But before he could register what was happening, all went dark.