There have been some reports on the appearance of craters from impacts into large bodies of water on Earth and some analogous craters on Mars. The main feature they show is that the craters are multiring, as is the crater in Meridiani. Another key feature is that they show resurge gullies. Note that a crater being multiring is not sufficiently diagnostic since these also occur on the Moon. However, resurge gullies might be diagnostic. The fluvial channels discussed in the "EVIDENCE FOR AN 800 KM DIAMETER IMPACT STRUCTURE IN MERIDIANI PLANUM" report might be examples of such resurge gullies.

Some reports on marine and lacustrine impacts:

IMPACT CRATERS AS INDICATORS FOR OCEANIC PHASES ON MARS.
J. Ormö1 and P. Muinonen2, 1International Research School of Planetary Sciences, Universita d’ Annunzio, Viale Pindaro 42, 65127 Pescara, Italy (ormo@sci.unich.it), 2Dept. of Physical Sciences/Astronomy, P.O.Box 333, 90571 Oulu, Finland (petri.muinonen@oulu.fi).
http://www.lpi.usra.edu/meetings/lpsc2000/pdf/1266.pdf

POSSIBILITIES AND SIGNIFICANCE OF MARINE-TARGET CRATERS ON MARS.
2004 Denver Annual Meeting (November 7–10, 2004)
Paper No. 81-4
http://gsa.confex.com/gsa/2004AM/finalprogram/abstract_74543.htm

Marine-target craters on Mars? An assessment study.
Meteoritics & Planetary Science, Vol. 39, No. 2, p.333-346
http://adsabs.harvard.edu/cgi-bin/bib_query?2004M&PS...39..333O

The Lockne Marine Impact in the Ordovician of Central Sweden.
Meteoritics & Planetary Science, vol. 31, page A80.
http://adsabs.harvard.edu/cgi-bin/bib_query?1996M&PSA..31R..80L

Recognition of marine-target craters on Earth, and its application to Mars.
American Astronomical Society, DPS meeting #31, #43.02.
http://adsabs.harvard.edu/cgi-bin/bib_query?1999DPS....31.4302O

Bob Clark