Complex sentence: Initially, it was actually lemon juice that was used as the additive to grog on the British Royal Navy ships, and at the time, the terms "lemon" and "lime" were used interchangeably to refer to citrus fruits.
Split into simpler sentences: Initially, it was lemon juice that was used as the additive to grog on the British Royal Navy ships. At the time, the terms "lemon" and "lime" were used interchangeably to refer to citrus fruits.