How old is your engine? I recently replaced the trim cylinder (not tilt - it is much harder) dust seals on my 1994 Yamaha 250 but in my case it was a slow leak - had to top off the tilt fluid resevoir every 3-6 months.
Not knowing the year of your engine, this might not be the exact picture but its pretty close. Note you can see and price the parts you will need here as well. The parts are cheap - its the labor that will kill you!
http://www.boats.net/parts/search/Yamah ... parts.html
I assume you are seeing hydraulic fluid coming out of parts 7/8 in the blowup?
The seals you see on the outside are really just dust seals - they are not the ones holding hydraulic pressure. So if you see fluid blowing out of the dust seal at a high rate (which is what it sounds like) that indicates you have a problem with the seal on the hydraulic piston itself.
To service the tilt (as opposed to the trim) cylinder seals you would
have to remove the whole tilt/trim assembly and dissassemble it on your bench. You can not get at the tilt seals with the unit on the engine. I rebuilt trim/tilt units on two different engines and would say it compares to rebuilding a carburator in the level of difficulty. The wildcard is how corroded your unit is and will you have to use a blowtorch to get out frozen bolts etc. In one case, using the proper spanner wrench, I could not get the cylinder end cap off even though it did not even appear corroded. Blowtorch and freeze spray had no impact. Good luck!
chris
