Firstly, you probably know you can turn the borders off.
Now what you have missed is what are called http://www.gamesas.com/index.php?/topic/1073491-wiprelz-under-the-sign-of-the-dragon-tamriel-heightmaps/ and the following http://www.gamesas.com/index.php?/topic/1080432-under-the-sign-of-the-dragon-tamriel-landscapes/ (very small by comparison).
What Onra has done is created and landscaped (though not extravagantly) the other provinces. They have little civilization to them and no encounters but the land is there to be modded. The idea being that we have been waiting for mod teams to produce entire province mods and they have taken upwards of 4 years with little output. Well why do an entire province? The land is now available to mod and yes you can sail all the way from Anvil around Summerset Isles to Layawiin and into Blackmarsh.
The northern provinces are slated to be released next - I think after Onra releases another version of Fort Akatosh.
This is mostly a beta type thing as the ships are super cheap (last I used it), but having tried all sailing mods this is the best by far.
There are other provinces available Valenwwod Improved and Skyrim improved are out and even though they have towns and ports the landscape is not finished like it is with Heightmaps - you could be trucking along and fall into hole that you did not see because the modders have not even finished getting the LOD to render correctly.
Tamriel is now much bigger.