While this is about midget submarines, but there's one other thing that caught my attention:Just it is really small that gives an upper hand in crowded shallow waters with its flexibility to pass through coastal areas or shallow straits between islands. Thus i always assumed Indonesia has been interested in these platforms.
Ever since Indo Defence 2016 when PINDAD and North Sea Boats introduced the "tank boat" project, I've been thinking along the same lines. Ofcourse, it's a different platform, but by having both surface and sub-surface combatants which are small, agile but still pack enough punch, it does seem that those places are where these two can be used most effectively.
The tank boat is for the army, though, so to be able to do what I just described above, there must be a tight coordination between the army and navy especially in those areas.
Unfortunately, although there was news that Indonesian MoD has ordered a number of it for trial in 2019, I have no idea how far the project has gotten by now - if it's still continues, that is.
Source.