The sea can be an enticing place. Fresh air wafting from the ocean, waves beating rhythmically, majestic... Unless you're in the presence of an Aoyama-Gosho character.
If you're with Shinichi someone, or multiple people, will certainly be murdered.
The same applies if you're near one dark-skinned Osakan, and the two of them together by the ocean usually spells "disaster".
If you're with a messy-haired magician, murder is unlikely, but things still won't resemble anything good. If you're lucky, you'll hear some ramblings about fish and be done with it.
Any-which-way, the sea can be wonderful... Unless you're under Aoyama-sama's control.