Sea and land bases don't interact via borders. The borders of land bases just split the difference basically. You can plant a base directly on the border of an enemy and it will scoot the border towards his base.
With no rival bases around, the borders go out quite far... something like 8 tiles, but it's easy to trim those back with rival bases being planted.
The way to protect land you want without fighting is to claim the area enough that the rival can't plant a base to move your borders. If you want a peninsula for example, put a city close enough to the end of it to claim all the available land so there is no where someone can put a base to move your borders.
To get your sea tiles back you need to remove/capture Miriam's base. Seas bases have very small borders (just their workable squares IIRC, maybe 1 extra ring) and they can't steal your land tiles.
(All of this is IIRC of course, but I think I do.)
With no rival bases around, the borders go out quite far... something like 8 tiles, but it's easy to trim those back with rival bases being planted.
The way to protect land you want without fighting is to claim the area enough that the rival can't plant a base to move your borders. If you want a peninsula for example, put a city close enough to the end of it to claim all the available land so there is no where someone can put a base to move your borders.
To get your sea tiles back you need to remove/capture Miriam's base. Seas bases have very small borders (just their workable squares IIRC, maybe 1 extra ring) and they can't steal your land tiles.
(All of this is IIRC of course, but I think I do.)