Water has a lower albedo than most land surfaces & especially ice, so if you were to hold constant the Earth's distance from the sun & axial tilt, etc. then the Earth with only islands would be much warmer.
Water also takes more energy to cool off and warm up as compared to land, so the climate would have less daily, annual and latitudinal variation. Increase in zonal avg. windspeed however would militate in favor of a heightened temperature contrast between the poles & the equator, so whether the latitudinal temperature contrast was increased or decreased would depend on the relative importance of heat transfer via atmospheric vs. oceanic circulation.
With very little land, the Earth may be too warm for even seasonal polar ice. It has been that way in the past after all (Cretaceous, Eocene) and the Earth still had significant exposed landmasses unlike in this scenario.
The Earth would also be warmer due to increased water vapor in the atmosphere from the higher temperatures (as the amount of water vapor in the air is dependent on the temperature). If the Earth became too warm, then at some point the stratosphere would become much more moist (as opposed to the current situation where it is essentially dry), and water from the Earth would be more easily lost to space. In all likelihood, this would eventually lead to the total loss of the world's oceans.