Even though I commented after Bruce earlier, I didn't see his response. So allow me to go back and address a few points...
For the most part the Med is not that deep.
The major issue with the Mediterranean is not the depth, but the relative clarity of the water. As a result, you can spot a submarine at periscope depth rather easily from an aircraft.
Most of the areas I'd identify for Greek submarine operations are actually pretty deep - deeper than the crush depth of the
Maklai design. For instance, the Ionian Sea is over 3,000 meters deep. The Aegean is much shallower, of course (some spots are quite shallow while others reach 400m), and the Adriatic is quite shallow indeed - although even the southern reaches of the Adriatic between Montenegro and Bari are 900m or more in depth. So I don't have any particular issues with a deep-diving submarine in the Mediterranean: it is entirely possible to reach this crush depth without bottoming out.
And on that point the crush depth seems quite excessive, but on technical grounds I’d defer to Brockpaine’s opinion. He knows Subsim far better than I do.
Crush depth in Subsim is... rather simplistic. If you have an oceanic submarine, you will always get one figure. If you design a coastal submarine, you get a second figure. There's no design input that will let you customize the crush depth. I list it in my writeups, but it's not a particularly accurate number and you'd best not treat it as gospel truth. Checking against real submarines, I'd guess Subsim gives you a crush depth about 125% in excess of what might be considered historical for the late 1940s.
The first question that comes to my mind is why Greece might require an oceanic-scale submarine, particularly one so large and with such long range, when presumably the Greek Navy would be operating in the Mediterranean, which historically has been troubled ground for large submarines. During the Second World War the Royal Navy found itself getting a better return on the smaller U and V class boats than with the larger S and T classes.
Yup - the U and V classes had much better dive times and maneuverability, which allowed them to avoid detection (and attacks) by aircraft more easily than the large fleet boats.
The French, who have the second-largest submarine fleet in the Mediterranean (after Italy), follow something of a multi-tiered approach overall. The French certainly have large submarines, but these are based in the Pacific or Atlantic fleets. The largest boats used for the Mediterranean are the
Daphne and
Thetis classes. The significantly more compact
Aréthuse class is coming into service as well, largely to replace older submarines.
French thinking on this matter favors small submarines, with their underwater mobility maximized at the cost of all other factors, including striking power. The
Aréthuse class is pretty indicative of this. They have four torpedo tubes and only one reload torpedo apiece - eight torpedoes in total. But their submerged speed is a knot higher than the most recent
Maklai variant, with 40% longer underwater range at creep speed.
Here's a go at a large coastal submarine, based on the French
Thetis-class, that France might propose for export to be license-built in Greece.
DCNS French Export Submarine, 800t Class, For Greece
Date: 1949
Type: Coastal
Length: 59.0m
Beam: 6.6m
Draft: 5.2m
Crush depth: 225m
Light Displacement 720t
Loaded Displacement 884t
Full Displacement 1012t
wt fuel&batts: 350t
Reserve buoyancy: 13%
Armament:
- 6 x 533mm torpedo tubes (all bow tubes)
- 24 tons for mines or twelve reload torpedoes
ElecHP: 3600hp
DieselHP: 1800hp
Speed:
- Max Surf Speed: 13.7 knots
- Max Sub Speed: 17.1 knots
Range:
- Surfaced: 6940nm@12 knots
- Submerged: 254nm@6 knots / 95nm@9 knots
Tons Oil: 140.0t
Tons Battery: 210.0t
Miscellaneous Weight: 39 tons
This is about the largest size that I (France) would really consider suitable for Mediterranean operations. It has plenty of endurance for long (relative to Mediterranean terms) patrols and a lot of reload torpedoes, but is not so large that it's going to be easy to spot and kill. Underwater performance (speed and range) is the crucial focus.