With more marginal anchors like brush and smaller rocks, you often need multiple anchors attached to a single carabineer or pulley - basically spreading the load. In this situation, try to avoid a vector pull which would really stress the carabineer. Keep the angle of pull from each anchor less than 90°. With multi-point loads, add a twist on one of the lines of webbing and hook the carabineer through to for a self-equalizing loop. Always consider what will happen if the loads moves (like the boat coming free). Will your anchor fail? Add an independent tag line for this possibility.
Here are two nice videos that demonstrate the concepts of load sharing versus load distributing multi-point anchor systems: