Befriend Mrs. Weeder
Mrs. Weeder is one of the first castaways you'll encounter on the island. She has a variety of seeds that she will be willing to trade you once you become her friend. To gain her affection, bring her gifts. Her favorites are coconuts, fruits, long beans, guacamole and veggie burgers. She is also in need of a scarecrow. Once you've won her over, she will trade seeds with you.
Find Blue Turquoise
To trade for the guava seed, you will need to give Mrs. Weeder one winkle and one turquoise coral. To obtain the coral, swim out to an area where coral is plentiful when you dive and search there. There is no way to tell or control what sort of coral you will bring back to the surface, so keep at it until a turquoise piece presents itself.
Find the Winkle
Once you have the turquoise coral, the hard part begins. Winkles are incredibly rare and hard to come by on the island. The only way to obtain one is to dive in the sea and dive until one turns up. Because of how uncommon the winkles are, this can take some time. If you are insistent on finding one, patience is the only way.
Use the Guava
Once you have the two items, you can trade them for the coveted guava seed. Once in your possession, plant the seed and begin harvesting the guava. Guava can be combined with sugar to make guava juice, a delicacy that other Sims are eager to trade for.