just sailing around the area may not be getting you close enough to see the crypts .put your key somewhere safe explore inside the swamps until you find 2 or 3 crypts near by then build a tree house using the large trees that can not take damage set up a portal to your main base and put some storage chests ready to store your iron scraps+ a bed to create a spawn point in case of death ,clear a path to the coast that allows you to pull a cart over to get your iron to your boat.if you don't want to risk a boat journey with your iron you have choice to use a mod that allows ore through portals some players hate using portals to transport ore other don't if your just playing solo play as you wish if joining a server for multiplayer then abide by their rules on this