There have probably been islands in Hillsborough Bay for almost as long as the bay has existed. The original delta islands were created as sediment, and other material flowed out of the Hillsborough River and into the bay. Plants eventually took root, and more material accumulated on the growing islands, more on the southerly island than on the one closer to the river's mouth.
Small sandbars extended out from the two islands, further demonstrating the shallow nature of Hillsborough Bay. It was these two islands, and the surrounding tidal area, that formed the nucleus for Seddon Island (now Harbour Island), created in 1904 as part of Tampa's growing port, and Davis Islands, developed by David P. Davis from 1924 to 1926 during Florida's land boom.
The islands first appeared, nameless, on early 17th century Spanish maps depicting Hillsborough Bay and Tampa Bay (then known as Bahia de Espirito Santo, or Bay of the Holy Spirit). The islands were later included as part of the Fort Brooke military reservation established in 1824, and it is probably during the fort years that the larger of the two islands picked up its first name, Depot Key. Other names, all describing a particular feature of the islands, appeared through the years, including Rabbit Island, Big and Little Islands, Grassy Islands and, eventually, Big Grassy and Little Grassy Islands.
The first recorded sale of either of the bay islands came on April 18, 1860, when William Whitaker purchased the southern tip of Depot Key (Big Grassy Island) from the government, a total of 61/3 acres, for $1 per acre. Little Grassy Island and the remainder of Depot Key were purchased in 1881 by a number of different interests.
William C. Brown purchased all of Little Grassy Island, totaling 161/3 acres, for $1 per acre. Brown and William B. Henderson partnered to purchase a large portion (693/4 acres) of Big Grassy Island from the state for 90 cents per acre. The town of Tampa purchased the remainder of the island, consisting of 281/2 acres, at the same price. Brown and Henderson, in turn, obtained a 99-year lease for the city's portion of Big Grassy Island for $20 a year.
During one of the first channel-dredging projects of the 1880s, cypress tree stumps were discovered in 8 feet of water a few yards south of Big Grassy Island. That revealed that the bay was a freshwater cypress swamp during the last Ice Age. Another channel-dredging project, begun in the early 1900s, bisected Little Grassy Island, creating Seddon Island on the east side of the channel and a remnant of Little Grassy Island on the west side. Little Grassy Island usually disappeared under a strong high tide, but Big Grassy Island generally remained dry. Both islands, however, were completely covered by water during the 1921 hurricane.
On June 8, 1920, Tampa City Council offered a referendum to voters asking whether they would support the purchase of Little Grassy Island for use as a city park. The referendum squeaked through at 694 to 692. Though non-binding, the city agreed with the majority and purchased Little Grassy Island from William Brown's widow, Mary E. Brown, on May 9, 1921, for $25,000 (more than $270,000 in today's dollars).
Many histories of Tampa and Davis Islands relate stories of Boy Scouts going to the bay islands for campouts. Tampa children made unsupervised forays to the islands, as well, including a young David Davis. In later years, Davis's brother, Milton, told interviewers that he and David ventured out onto the scrub-covered mud flats in the late 1890s, catching crabs and frying fish instead of attending school. As an adult, Davis found another use for the bay islands.