If a spiderden spawns or is placed directly at the shore, the spidercreep can spawn on ocean tiles. Presumably the tile check happens in the function that spreads creep, but not the one that starts it.
GIF showing the problem:
Steps to Reproduce
1) Take spidereggsack 2) Place as close to the shore as possible
This issue has been fixed in either a future version or updated in a previous version.