I don't think that I would use a rubber washer under a keel bolt. Keel bolts need to be torqued tp a very high tension. They need to maintain this tension through a pretty wide range of loadings with negligible elongation. That would discourage the use of a compressible washer. If a comperssible washer had to be used, I would think that urethane washer would be the best material for that purpose. Urethane can be specified in a range of stiffness, and I would guess that the least compressive urethane might work. Urethane can be ordered in a formulation that is stable with prolonged exposure to water. With some Urethanes are some issues when exposed to petrochemicals for prolonged periods as might accidently happen in a bilge.