I'm anchoring an 18 foot ski boat near a rocky lake shoreline. I'm using an
Anchor Buddy (bungee add-on for the rode) to hold the boat off shore. I'm
using 3/8" 3 strand nylon to tie to a tree on land to keep the boat near
shore, but not on shore.

AnchorBuddy.com if you need a video to see what I'm talking about.

Right now I'm using a 100 foot line to attach the bow to the tree. I tied a
bowline knot at the tree end.

Is the bowline a good knot that will withstand some gentle tugging all night
long?

If I wanted to carry 2 50 foot sections of line for tieing to the tree
insted of the single 100 foot section I might occasionally need to tie the 2
sections together to reach the tree. Is a sheet bend a good knot for
holding the 2 sections together all night, again with the stress of gentle
tugging? Or should I attach the 2 sections to each other using bowline
knots?