Answer
Decrease.
Work Step by Step
The formula for double slit interference patterns is given by $dsin \theta=m \lambda$. For small angles, $sin \theta \approx tan \theta \approx \theta$. Therefore, $sin \theta$ can be rewritten as $\frac{y}{L}$. This leaves $$\frac{dy}{L}=m \lambda$$ Assume $m$ (order number) is one and use $y$ as the distance from the center to the next area of maximum brightness. This leaves $$y=\frac{\lambda L}{d}$$ If the apparatus is submerged in a different surface, wavelength decreases. Therefore, the distance between fringes would decrease.
