You just need to imagine how large of a planet that is surrounding it, or how small the other planet could be. Who says what the limits of size are in deep space? For all we know, our universe is but a mere water droplet of another universe much larger than we can comprehend.
Oh, I can perfectly fathom a planet this comparatively large to another, just not within striking distance. Current science doesn't support the idea that two celestial bodies of this size difference could orbit the same star long enough for the smaller one to be considered a planet, then collide; or even that it would retain its shape upon entering the larger planet's atmosphere.
Either way, a planet is by definition a celestial body that orbits a star, so an object submerged in a planet's ocean could not be called a planet.
Crazy! Makes me wonder what it would be like if Earth shot off one day and landed on another Ocean Planet! Hopefully the people on the side that doesn't hit the ocean Survive! Perhaps Earth would just become another Island on the Gigantic Ocean Planet...