Hey man, thank you for the very kind words!
For the purpose of the jam, maybe I should have let every port allow upgrades, but in the capital's shop on Jerico you can press tab to enter the shipyard, where you can buy upgrades.
Glad you liked the game! There's a lot of options to go further with this, and we're currently consolidating them. More islands are a must, and I would love to see some island development.