The main reason wood rots is because it is "sap wood", not heart wood. Ever notice some wood has gray areas? Those gray areas are sap wood, wood which was near the surface of the log, hence, sap wood.
You can buy wood for trim that is fingerjointed. While it is relatively inexpensive, it is not economical because much of it is sap wood. This trim wood should never be used for structural construction members. Even some pressure treated lumber contains sap wood, and it will rot sooner than pressure treated heart wood.
It may be illegal in some places to use pressure treated wood for bunks or other uses that bring it into contact with ponds/lakes/rivers.
There are two types of boards, plain sawn and quarter sawn. In plain sawn lumber, say a two by four, the growth rings will be oriented along the four inch dimension. In quarter sawn, they will be oriented more or less parallel to the two inch edges.
I'd recommend the quarter sawn. While it can contain sapwood, it will only be along one edge. If you see gray, stay away. One other note. Plain sawn lumber is more likely to warp, cup, or distort. Quarter sawn lumber is more "stable" because wood expands and contracts across the growth rings. In quarter sawn when the wood expands, it gets wider. In plain sawn, it cups, warps, twists, bows, etc.
Since the idea is to distribute support for the hull evenly along the bunk, the quarter sawn will do that better than plain sawn lumber.
"The outer, lighter colored wood is the sapwood. This is the "working" part of the tree, as water and sap will flow through the sapwood much like blood through your arteries, veins and capillaries. While this part of the trunk is vital to the tree when it is living, it doesn't make for very good stock for woodworking. Sapwood contains a lot of moisture, will shrink considerably when dried, and is much more susceptible to fungus.
The inner, darker section of the trunk is the heartwood. Heartwood is formed from old, "retired" sapwood, and becomes the strong spine of the tree. Heartwood is preferred for woodworking, as it is far less susceptible to fungus and doesn't contain nearly as much moisture as sapwood, which means it will shrink less when dried. Many experienced woodworkers will actually remove the sapwood and use only heartwood on their furniture projects.
Once the tree has "promoted" some of it's sapwood to heartwood status, the sap will stop flowing through that part of the wood and the converting material essentially dies. As part of the conversion process, the pores will begin to plug up with organic matter which causes the cell walls to change color due to the presence of chemicals called extractives. The extractives are responsible for the rich character and colors found in heartwoods.
Should Sapwood be Discarded?:
Just because you end up with some excess sapwood after trimming it from the hardwood when you begin a furniture project, does that mean that the sapwood is "junk?"
My answer is no. While the sapwood will never be as strong, rich or beautiful as the heartwood, it still has its uses. Just be certain to dry the sapwood thoroughly, and use it in projects where a little bit of movement will not cause problems, and where it will be thoroughly sealed (with paint or polyurethane) when finished."
http://woodworking.about.com/od/typesofwood/p/Heartwood.htm
