Looking on the internet deeply has found these results:

in the seaside is the most popular phrase on the web.

by the seaside 11,240,000 results on the web	in the seaside 15,840,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
In summer months the situation becomes unbearable, especially at weekends, when the road is completely paralysed by the traffic congestion caused by people travelling to/from the seaside near Rome. But since I'm living by the seaside I should take up painting. And it would be a nice way for us to spend a day by the seaside. But if you want to know, l was born by the seaside in Whitstable, where the oysters come from. But since I'm living by the seaside It's that painting at the mansion, the seaside of Italy. Nothing better than an afternoon by the seaside. Since I live by the seaside, maybe I should try painting.	It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

by the seaside

11,240,000 results on the web

in the seaside

15,840,000 results on the web

More popular!

In summer months the situation becomes unbearable, especially at weekends, when the road is completely paralysed by the traffic congestion caused by people travelling to/from the seaside near Rome.
But since I'm living by the seaside I should take up painting.
And it would be a nice way for us to spend a day by the seaside.
But if you want to know, l was born by the seaside in Whitstable, where the oysters come from.
But since I'm living by the seaside
It's that painting at the mansion, the seaside of Italy.
Nothing better than an afternoon by the seaside.
Since I live by the seaside, maybe I should try painting.

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.