Map A. Sea Depths The shallowness (less than 200 meters) of water surrounding most Southeast Asian nations has, to a degree, limited the takeup of submarines. Shallow water is often encumbered by reefs, many islands and larger landforms that define straits and narrows. There is also heavy ship traffic through many of the straits. These hazards are difficult and dangerous for submarines to move through. (Map courtesy US Centers for Environmental Information).
A third Improved 209, KRI Nagaransang (405), is likely to be built at PT PAL (Surabaya), Java, Indonesia and perhaps commissioned in 2020 or later.
There have been many reports over the years that Russian and/or DCNS (French) submarines are also being considered. It would probably make much more sense to build additional 209s at PT PAL.
Map B. the Straits - If the 2.8km narrow and 25m shallow Malacca Strait were blocked the Sunda Strait, as shallow as 20m, is inadequate to take up much ship traffic. The Lombok Strait, 20km wide and up to 250m deep is better proportioned to handle large ships.
Disagreements between Indonesian factions are likely to exist behind the presidential visions. There are likely to be many attitudes in the Indonesian Navy, broader defence and industry development bureacracies about how many more than 2 or 3 submarines can be bought, maintained and crewed.
At one end of the spectrum Indonesia may only have one crew to operate just one of the current Type 209s (KRI Cakra (401) or KRI Nanggala (402)) at any one time. Meanwhile the nucleus of another crew may be in South Korea training to use the first Improved 209 (KRI Nagabanda (403)).
Another faction may want the 3 Improved 209s + two extra subs (for a total of five).
Others, seem to want a total of twelve submarines by the late 2020s for example 3 Improved 209s + 9 other subs (of Russian or French origin). Such a faction may be of submarine service origin because if the Indonesian Navy operated twelve submarines there may be no money or men left in the Navy for major surface ships.
Twelve might be justified by: expanding strategic responsibilities; Indonesia’s rising GDP enabling it to buy more; a return to Indonesia’s submarine glory days when it had twelve Russian supplied Whiskey class submarines (late 1950s to mid 1960s