This has been a recurent question in the General Forum but I thought this would rather belong here. So here it goes:
1. Straits are defined in two different files: \Map\adj-defs.csv contains the strait's *code*, e.g.:
and \DB\straits.csv contains the strait's *graphics*, e.g.:
where each couple of numbers stands for the coordinates of one little rock in the sea.
2. To remove a strait, just delete the corresponding line in adj-defs.csv which is easy because there are comments, and the corresponding line(s) in straits.csv which is tricky because there aren't any comments.
To help people out, here is a commented file that I found on the forum soon after the beta patch making straits moddable was released:
3. To add a strait, you have to add a line in adj-defs.csv which is easy because you can find provinces and sea provinces IDs in province.csv, and as many lines as you want rocks to appear on your map in straits.csv, which is not so easy because you have no idea as to how the coordinates fit on the map.
Here are some I added myself. It is rather a tedious work to find the right numbers for the right place so this may save you some time if you thought about adding them yourself:
Feel free to add some more if you already made them. 
1. Straits are defined in two different files: \Map\adj-defs.csv contains the strait's *code*, e.g.:
Code:
395;394;sea;998;Messina-Apulia
Code:
10186;2742
10194;2752
10182;27582. To remove a strait, just delete the corresponding line in adj-defs.csv which is easy because there are comments, and the corresponding line(s) in straits.csv which is tricky because there aren't any comments.
To help people out, here is a commented file that I found on the forum soon after the beta patch making straits moddable was released:
Code:
9624;1474
9620;1482 #Holland-Friesen (337 to 339)#
9630;1490
10794;2434
10782;2438 #Thrace-Smyrna (357 to 478)#
10780;2447
10186;2742
10194;2752 #Apulia-Messina (394 to 395)#
10182;2758
11150;2040
11148;2048 #Kaffa-Kerch (464 to 465)#
11166;2045
16474;2909
16490;2904
16502;2899 #Shikoku-Kansai (686 to 687)#
16488;2917
16502;2910
10892;2386
10906;2395 #Thrace-Anatolia (357 to 477)#
10890;2399
4928;1176
4934;1187
4940;1198 #Sault-Nipigon (93 to 95)#
4948;1209
10646;2589
10660;2595 #Ionia-Hellas (479 to 359)#
10648;2602
10788;2587
10802;2593 #Ionia-Smyrna (479 to 478)#
10786;2601
10700;2450
10714;2456 #Smyrna-Macedonia (478 to 358)#
10698;2462
9892;1300
9918;1302 #Sjaelland-Jutland (307 to 309)#
9904;1306
10025;1370
10015;1360 #Sjaelland-Skane (307 to 251)#
10015;1380
9334;1594
9339;1625 #Kent-Calais (248 to 382)#
9330;1610
9350;1638
9025;1270
9018;1281 #Ulster-Strathclyde (232 to 239)#
9030;1290
8763;2762
8781;2756 #Gibraltar-Tangiers (445 to 732)#
8769;27503. To add a strait, you have to add a line in adj-defs.csv which is easy because you can find provinces and sea provinces IDs in province.csv, and as many lines as you want rocks to appear on your map in straits.csv, which is not so easy because you have no idea as to how the coordinates fit on the map.
Here are some I added myself. It is rather a tedious work to find the right numbers for the right place so this may save you some time if you thought about adding them yourself:
Code:
397;398;sea;992;Sardinia-Corsica
9786;2392
9772;2395
820;361;sea;1001;Corfu-Albania
10499;2627
10508;2621
820;359;sea;1002;Corfu-Hellas
10516;2655
10528;2650
10540;2645
116;117;sea;864;Nova Scotia-Isle Royale
6175;1368
6183;1361
6194;1356
118-124;sea;833;Gander-Belle-Isle
6490;955
6482;947
6499;946
151;178;sea;1045;Tobago y Trinidad-Caribe
5860;3507
5874;3518
228;229;sea;1241;Magallanes-Tierra del Fuego
5698;6770
5686;6767
5673;6764
228;229;sea;1239;Magallanes-Tierra del Fuego
5623;6815
5628;6823
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