Hi nickib,
Not bother AT ALL! This stuff is why we are ALL here. It would be a fairly boring site if no one ever asked any questions :~)
There are 5 places that water can enter the lower end because of sealing problem.
2 of these are the case drain plug (or screw) and the case fill plug. They have tiny Orings on them that can dry out or the screw itself can become loose. Pretty simple. However, I could NOT find an illustration or a part number for these Orings. They should be fairly easy to find in any competent, well stocked hardware store though.
THEN, things get much more complicated with the other 3 places.
These are:...the propshaft seal.....the shiftshaft seal......and the mainshaft seal. Below is a link for the transmission parts page. If you look at item #21, that is the prop shaft seal. It is the most COMMON place for failure due to fishing line wrapping behind the prop...as I've pointed out already. It is also the EASIEST of the three seals to replace. Although, I can tell you from experience, that it is easy to booger one up on installation too if you are inexperienced or don't have the proper tools. The easy part is that you can remove item #11, the "prop shaft holder" from the "bullet" or trans case and work on it on a bench. You may also note that the prop shaft holder is sold as an assembly and includes the seal as well as new bearings. The assembly is not killer expensive and is sometimes what I recommend, as an alternative to digging out the old seal and pressing in a new one, to someone not skilled in mechanics.
http://www.boats.net/parts/search/Honda/Outboard Engine/1998/BF15AW SA/PROPELLER SHAFT/parts.html
Next comes the shift shaft seal...or seals. In the link below find items 13, 27 and 28. You will need these three to seal that area should you determine that is where the water is entering. With all your shifting issues, it is a distinct possibility that something is... let's use the term...awry....in that area on your outboard. Inspection there may have you "killing two birds" as it were.
http://www.boats.net/parts/search/Honda/Outboard Engine/1998/BF15AW SA/VERTICAL SHAFT/parts.html
And then we come to the mainshaft seal. In the link below it is item #28. To replace this seal it is required that the gearcase be pretty much disassembled (except for the lower shift shaft) and the mainshaft removed. Not really all that bad of a job, but not for a novice or totally inexperienced mechanic unless you have gobs of time and really good organizational skills to keep all the parts laid out and not get them (or yourself) mixed up. Just a lot of holding your tongue "just so" if you know what I mean.
http://www.boats.net/parts/search/Honda/Outboard Engine/1998/BF15AW LAS/EXTENSION CASE/parts.html
Also, I've included a link to the FRONT page of the boats.net Honda parts webpage. On that page is an index where you can locate an approximation of your outboard...all American models as far as I know but most of this stuff is pretty much the same from what I can tell. Also, there is a button at the top of the page just right of center that says "INTERNATIONAL" and next to that is another button that says "SHIPPING". Hopefully you can find what you need here and get it shipped without taking out a mortgage on your home!
http://www.boats.net/parts/search/Honda/Outboard Engine/parts.html
Again, ask ALL the questions you want or need to......WE LOVE IT!
Good luck