there's a few things with the sea level that make changing it not really useful.

1) the actual floor levels don't seem to change with it, meaning setting it to above 63 will always result in endless oceans while below makes the oceans not appear, making this setting pretty useless without a land level to accompany it.

2) setting it to above 127 crashes the game with this error:

java.lang.IllegalArgumentException: bound must be positive
at java.util.Random.nextInt(Unknown Source)
at ayy.a(SourceFile:442)
at od.a(SourceFile:175)
at axr.a(SourceFile:859)
at od.c(SourceFile:104)
at od.d(SourceFile:117)
at aky.b(SourceFile:289)
at aky.f(SourceFile:284)
at aky.c(SourceFile:220)
at aky.d(SourceFile:235)
at aky.b(SourceFile:201)
at ayj.a(SourceFile:68)
at oe.b(SourceFile:637)
at oe.a(SourceFile:595)
at cim.a(SourceFile:80)
at cim.g(SourceFile:110)
at net.minecraft.server.MinecraftServer.run(SourceFile:367)
at mu.run(SourceFile:667)