Maybe this ER diagram will get you started:
dbdiagram.io - Database Relationship Diagram[
^]
You can hover over the diagram relation links to see if they are one-to-one or one-to-many.
Can not tell exactly how this diagram is supposed to work as I did not create it.
The SQL code:
CREATE TABLE "sailors" (
"id" SERIAL PRIMARY KEY,
"full_name" varchar(50),
"created_at" timestamp
);
CREATE TABLE "reservations" (
"id" SERIAL PRIMARY KEY,
"day" datetime,
"sailors_id" int,
"boats_id" int
);
CREATE TABLE "boats" (
"id" int PRIMARY KEY,
"name" varchar(50)
);
CREATE TABLE "crew" (
"id" int PRIMARY KEY,
"name" varchar(50)
);
CREATE TABLE "leads" (
"crew_id" int PRIMARY KEY,
"sailors_id" int
);
CREATE TABLE "belongs" (
"crew_id" int PRIMARY KEY,
"sailors_id" int
);
ALTER TABLE "sailors" ADD FOREIGN KEY ("id") REFERENCES "reservations" ("sailors_id");
ALTER TABLE "boats" ADD FOREIGN KEY ("id") REFERENCES "reservations" ("boats_id");
ALTER TABLE "leads" ADD FOREIGN KEY ("sailors_id") REFERENCES "sailors" ("id");
ALTER TABLE "belongs" ADD FOREIGN KEY ("sailors_id") REFERENCES "sailors" ("id");
ALTER TABLE "crew" ADD FOREIGN KEY ("id") REFERENCES "leads" ("crew_id");
ALTER TABLE "crew" ADD FOREIGN KEY ("id") REFERENCES "belongs" ("crew_id");