It depends on if it was treated or not... Wolmanized, pressure treated or if it is just good quality Marine grade.
Wolmanized;
5. Is Wolmanized Outdoor wood safe?
A comprehensive study of occupational, residential, and playground uses of wood pressure-treated with copper azole preservative has concluded, "no adverse health effects are expected." Believed to be the first independent human health risk assessment of the new generation of treated wood products, the study was commissioned by Bayer Chemicals Corporation (now Lanxess Corporation) and conducted by Gradient Corporation, a noted environmental and toxicological consulting firm. Lanxess manufactures Preventol® A8, which is an azole fungicide used in the copper azole preservative.
A number of different exposures (e.g., inhalation, incidental ingestion, exposure to the skin) were assessed and aggregated in each of these scenarios. The highest potential risk was estimated to be 17 times lower than the level that the EPA uses as a safety benchmark, thus demonstrating the safety of copper azole-treated wood. View the report summary (PDF).
Pressure treated;
This wood has been preserved by pressure-treatment with an EPA-registered pesticide containing inorganic arsenic to protect it from insect attack and decay. Wood treated with inorganic arsenic should be used only where such protection is important.
Inorganic arsenic penetrates deeply into and remains in the pressure-treated wood for a long time. However, some chemical may migrate from treated wood into surrounding soil over time and may also be dislodged from the wood surface upon contact with skin. Exposure to inorganic arsenic may present certain hazards. Therefore, the following precautions should be taken both when handling the treated wood and in determining where to use or dispose of the treated wood.
Marine grade Plywood;
Marine-grade plywood is a specially designed panel made entirely of Douglas-fir or Western Larch. The grade of all plies of veneer is B or better, which means it may have knots, but no knotholes. The panels are sanded on both faces, and are also available with Medium Density Overlay (MDO) or High Density Overlay (HDO) faces. The maximum core-gap size permitted is 1/8 inch. Its exposure durability rating is EXTERIOR and the glue used is a fully waterproof structural adhesive. It is considered a “premium” panel grade for use in situations where these characteristics are required, i.e., for boat hulls and other marine applications where bending is involved.
Marine-grade plywood is not treated with any chemicals to enhance its resistance to decay. If decay is a concern, it should be pressure-preservative treated to an appropriate standard.
So if it is not treated I would use it in a heartbeat.
