This order is represented by the family Gaviidae with the genus Gavia as its only member. 

Species-level timetree of extant Gaviidae based on Sprengelmeyer (2014), with the distribution of each family being indicated by the colour-code used throughout this website (Distribution code). 

References

Hemmer J (2020), Red-throated Diver: Gavia stellata, (2. edition), Books on Demand, Norderstedt. (link)

Moon JI, Park JG, Nam DH, and Lee DH (2019), Complete mitochondrial genome of the Pacific Loon, Gavia pacifica (Gaviiformes: Gaviidae), Mitochondrial DNA Part B 4, 1200-01. (pdf)

Sprengelmeyer QD (2014), "A phylogenetic reevaluation of the genus Gavia using next-generation sequencing", Master´s Thesis, Northern Michigan University. (pdf)