Haha, I took this above ocean challenge a bit too far. I've decided I'd make an engraved obsidian fortress casted over the deep ocean soil layers, it took a while to find the perfect embark site. It's 3x6 for fps reasons. If someone can point me to an image hosting site, I'll post screenshots of the embark location.
These parameters are for vanilla DF make about 3-4 suitable seaside locations per gen (just an estimate), and about 1 deep sea soil location about once per 3-4 gens.
My embark location is 9 down and 3 across on the world map, directly left of a human town. It is sedimentary on the shore, so bauxite and magnetite should be there, though I haven't any while setting up my basic camp.
I haven't finished a re-gen yet so this might not be it, but I'm 95% sure.
[WORLD_GEN]
[TITLE:OCEANBORN]
[SEED:1478178918]
[NAME_SEED:1048687696]
[DIM:257:257]
[END_YEAR:1050]
[BEAST_END_YEAR:200:60]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:1]
[ELEVATION:1:400:1600:1600]
[RAINFALL:0:100:400:400]
[TEMPERATURE:25:75:400:400]
[DRAINAGE:0:100:400:400]
[VOLCANISM:0:100:1600:1600]
[SAVAGERY:0:100:400:400]
[ELEVATION_FREQUENCY:1:50:4:15:4:30]
[RAIN_FREQUENCY:1:1:1:3:2:2]
[DRAINAGE_FREQUENCY:1:1:2:2:2:1]
[TEMPERATURE_FREQUENCY:1:1:3:9:5:1]
[SAVAGERY_FREQUENCY:1:1:2:2:2:1]
[VOLCANISM_FREQUENCY:1:15:3:12:2:20]
[GOOD_SQ_COUNTS:700:6500:4000]
[EVIL_SQ_COUNTS:700:6500:4000]
[PEAK_NUMBER_MIN:0]
[OCEAN_EDGE_MIN:0]
[VOLCANO_MIN:100]
[REGION_COUNTS:SWAMP:0:0:0]
[REGION_COUNTS:DESERT:0:0:0]
[REGION_COUNTS:FOREST:0:0:0]
[REGION_COUNTS:MOUNTAINS:0:0:0]
[REGION_COUNTS:OCEAN:0:0:0]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:0:0:0]
[REGION_COUNTS:HILLS:0:0:0]
[EROSION_CYCLE_COUNT:120]
[RIVER_MINS:400:400]
[PERIODICALLY_ERODE_EXTREMES:1]
[OROGRAPHIC_PRECIPITATION:1]
[SUBREGION_MAX:3500]
[CAVE_MIN_SIZE:5]
[CAVE_MAX_SIZE:45]
[MOUNTAIN_CAVE_MIN:100]
[NON_MOUNTAIN_CAVE_MIN:200]
[ALL_CAVES_VISIBLE:1]
[SHOW_EMBARK_RIVER:2]
[SHOW_EMBARK_POOL:2]
[SHOW_EMBARK_M_POOL:2]
[SHOW_EMBARK_M_PIPE:2]
[SHOW_EMBARK_CHASM:2]
[SHOW_EMBARK_PIT:2]
[SHOW_EMBARK_OTHER:2]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:65]
[TOTAL_CIV_POPULATION:20000]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:0:0:0]
[RAIN_RANGES:0:0:0]
[DRAINAGE_RANGES:0:0:0]
[SAVAGERY_RANGES:0:0:0]
[VOLCANISM_RANGES:0:0:0]
If you're really keen about finding the perfect embark spot you'll probably want to figure out how to make hundreds of little oceans, I've seen it done with mountains.