water mask

open the file that looks like
Area_Lp1_SnapOff_N039292327_N039250303_E002564668_E003010692.bmp
in PS then save it as tif
you'll then be able to add alpha channels to it
add an alpha channel (channels pallete, next to layers) and paint where the land is in white and the sea in black.
save it then all you have to do is open the inf file in notepad and alter these two lines

[Source]
Type = Custom

to this

[Source]
Layer = Imagery
Type = tiff

oh and remember to change .bmp to .tif on the filename reference

---

I created the same tile you did using fsearthtiles
that gave me two files;

Area_Lp1_SnapOff_N024335088_N024333466_W081490186_W081483741.bmp
AreaFSInfo_Lp1_SnapOff_N024335088_N024333466_W081490186_W081483741.inf

I opened the bmp in photoshop
saved as tiff
added two alpha channels (water and opacity)

then edited the inf
original inf-----------------------------------
[Destination]
DestDir = D:\fsx scenery 2009\res 4\scenery
DestBaseFileName = Area_Lp1_SnapOff_N024335088_N024333466_W081490186_W081483741
BuildSeasons = 0
UseSourceDimensions = 1
CompressionQuality = 100

[Source]
Type = Custom
SourceDir = D:\FSEarthTiles\work\res 4
SourceFile = Area_Lp1_SnapOff_N024335088_N024333466_W081490186_W081483741.bmp
Lon = -81.8171882629395 ;for top left and bottom right is: -81.8103790283203
Lat = 24.5641422271729 ;for top left and bottom right is: 24.5596253871918
NumOfCellsPerLine = 476 ;Pixel is not used in FSX
NumOfLines = 421 ;Pixel is not used in FSX
CellXdimensionDeg = 1.43051147460938E-05
CellYdimensionDeg = 1.07288360595703E-05
PixelIsPoint = 0
SamplingMethod = Point
-----------------------------


new inf-------------------------------
[Destination]
DestDir = D:\fsx scenery 2009\res 4\scenery
DestBaseFileName = Area_Lp1_SnapOff_N024335088_N024333466_W081490186_W081483741
BuildSeasons = 0
UseSourceDimensions = 1
CompressionQuality = 100

[Source]
Layer = Imagery
Type = tiff
SourceDir = D:\FSEarthTiles\work\res 4
SourceFile = Area_Lp1_SnapOff_N024335088_N024333466_W081490186_W081483741.tif
Lon = -81.8171882629395 ;for top left and bottom right is: -81.8103790283203
Lat = 24.5641422271729 ;for top left and bottom right is: 24.5596253871918
NumOfCellsPerLine = 476 ;Pixel is not used in FSX
NumOfLines = 421 ;Pixel is not used in FSX
CellXdimensionDeg = 1.43051147460938E-05
CellYdimensionDeg = 1.07288360595703E-05
PixelIsPoint = 0
SamplingMethod = Point
------------------------------------

the result was a bit rough, I rushed it a bit, but you get the idea