Multifunctional structure-battery composites were developed using fiber reinforced marine composites for structure function and rechargeable lithium-ion cells for energy storage and structure function. Laminate, sandwich, and modular beam configurations were fabricated and tested to determine flexural stiffness and strength, energy storage capacity versus discharge rate, and buoyancy (density). The structure-battery composites exhibited higher flexural stiffness but lower strength than equivalent unifunctional designs, energy storage capacities between 40 and 60 Wh/L, and buoyancies bracketing the unifunctional specimen values. Issues requiring further attention include: improved bending strength, simplified fabrication, reversible attachments for modular components, electrical wiring and connections, and battery management circuitry.