you can set your altitudes to whatever you want in the MapTrans.txt within the Scripts folder of dragon.
Code:
// (02) water
20 02 -5 00a8 00a9 00aa 00ab 00a9 00aa 00ab
21 02 048 00a8 00a9 00aa 00ab
22 02 010 00a8 00a9 00aa 00ab
23 02 015 00a8 00a9 00aa 00ab
a7 02 020 00a8 00a9 00aa 00ab
a8 02 025 00a8 00a9 00aa 00ab
a9 02 030 00a8 00a9 00aa 00ab
aa 02 035 00a8 00a9 00aa 00ab
ab 02 040 00a8 00a9 00aa 00ab
ac 02 045 00a8 00a9 00aa 00ab
ad 02 003 00a8 00a9 00aa 00ab
the 3rd entry # is the altitude (mine are modified)
setting it at -80 would cause no troubles at all, so long as you have the land mass it is going into transition correctly. the client will preform just fine at that level. the altitude the client works is -127 to 127. When your character hits 127, 128z is not 128, but actually a loop back to -127. so there is no 200 z level as Stormcrow stated. Depending on what EMU you use, you might have to adjust some scripts for the server to allow boats in that altitude of water. Some wont have any trouble, others will.