What you have implicitly done is to create a single preferred frame of reference, the one in which the sailboat's velocity and the velocity of the beam of light are referenced. That is very contrary to relativity theory.
The ball is just a distraction here, as is gravitation. Instead of that sailboat with an impossibly tall mast, imagine a spaceship with that same impossibly long mast. Our spaceship is far removed from any gravitational sources.
Suppose a member on the ship's crew goes to the end of that mast, stops with respect to the mast, and aims a pulse of laser light at the base. The laser will of course hit the base of the mast. From this crew member's perspective, it will take one second for the light to travel from the end of the mast to the base.
Suppose some other crew member is flying around outside the spaceship, such that the spaceship appear to be moving at a velocity $\boldsymbol u$ orthogonal to the mast that is orthogonal to the mast. This is analogous to your situation where the sailboat is moving.
This other crew member will also see the laser pulse hit the base of the mast. The magnitude of the pulse of laser light is c from the perspective of this other crew member; the speed of light is a universal constant. However, from the perspective of this other crew member, the direction of that laser pulse is not directly toward the base. The velocity vector of that laser pulse instead has a component orthogonal to the mast, equal to $\boldsymbol u$. The component along the mast is a bit less than $c$.
In relativity, velocities don't add linearly as they do in Newtonian mechanics. You need to use the relativistic velocity addition formula to compute the composition of two velocity vectors.
If you use this formula to calculate the velocity of a pulse of light emitted by a source moving at some velocity $\boldsymbol u$ (where $u = ||\boldsymbol u||$ is less than $c$), you will find that the magnitude of the light pulse is always $c$, regardless of the source's motion, and regardless of the direction in which the pulse was emitted.