Factorio's Island map preset gives you an island, but nothing to find beyond that - just infinite ocean. Island Start offers an alternative: There are other islands and continents out there.
This mod uses the built in mapgen system, so generation is fast - as fast as vanilla.
Modes
Island Start has a few different modes you can use - all available as presets, but you can also apply them to other presets (such as marathon, deathworld, etc) by going to the "Terrain" tab and selecting one of the "Map Types" with 'IS' in the name.
Fixed and Scaling Island
The first two modes give you waterworlds - lots of ocean, similar in spirit to vanilla's island mode, except that there are other landmasses out there you can find instead of only infinite ocean. "Fixed-sized" island mode lets you scale these other lands (via the standard map settings) up or down without affecting the size of your island, while the scaling mode makes your starter island affected by the map setting.
Tip! Want a lot of small islands? Try these settings:
Don't be afraid to reduce the scale slider -
Ribbon Lake Mode
This mode uses Factorio's default terrain generation - so generally a lot of land, unless you crank up the water settings - but it generates a lake that surrounds your starting location. In this mode, an absence of land connections is guaranteed by the map generator.
You can increase the size of your island by going to the "Enemies" tab of the map generator and increasing the "Starting Area Size" slider. The lake will not scale under 100% to avoid cutting you off from your starting resource spawns.
Attention!
To ensure you can make enough landfill to reach other lands, I recommend increasing the resource settings for Stone.
There's also a variety of mods out there that add ships, bridges, and aircraft - if you are playing on a waterworld, it may be a good time to take a look at these!
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Lastly, there's a just-for-fun mode in the settings - You can change any preset/map type into an island start by checking "Enable Direct Placement Mode". However, this mode looks far less natural: it appears as a circular moat instead - but it can also be used to create a giant wall.