Complex sentence: Rough seas and high winds meant that both American and Spanish rounds were being blown off target, which resulted in a severe disadvantage for the Spanish gunners, as well as resulting in several stray rounds falling on San Juan houses or other buildings.
Split into simpler sentences: Rough seas and high winds meant that both American and Spanish rounds were being blown off target, which resulted in a severe disadvantage for the Spanish gunners. This resulted in several stray rounds falling on San Juan houses and other buildings.