Depends on the conditions, nature of sediment, budget, and over what period you want to know the sed rate. For longer timescales and muddier sediment, you could take a core of the sediment (freezer core preferred to be sure you get the mud/water interface) and date horizons in the mud (probably lead-210 or C-14, depending sed rate and nature of sed). You may have to deal with sediment compaction and loss of organic material (see Campbell, 1996, power function for interpolating dates in recent sediment, J. Paleolim 15:107-110). For sandier sediment in a nearshore environment, you will probably be dealing with highly variable sed rates over time including periods of negative sedimentaion (erosion), so knowing the time-sclae you want to look at for sed rate (single storm event? "typical" calm day? seasonal? annual? decadal?) will be critical.

Thanks a lot Ian. I am interested in the sedimentation rates from the water column to the bottom. We only have one month to measured, but I would like to know the rate of this month (I know that it will be highly variable, it is antarctic waters), but I would like to have an approximate value of this rate, nothing rigorous. 

The problem ius that almost any device you might install has the potential to affect sedimentation pattersd around it if there is any turbulence at all in the environment. Is the bottom sandy, silty, clayey or organic? If is it very fine grained, then a simple can embedded in the sediment collect after a period of time would give you a rough estimate, particularly if you do a few of them. In coarser sediment you could also try doping the surface fo teh sediment with a marker - maybe colored glass ground to the same grain size as the sediment - and then take a very short core at the end of the period to measure how deeply that doped layer was buried. Do you know what the bottom sediment is like? 

Yes, there are different sizes with poor sorting, from coarse silt to medium sand, but the most important matter is that this bottom are steepy. The input of this sedimentation came from the outside of the water due to the wind inflow (only in summer, in winter it is covered by snow). It is an active volcano, then there is a lot of ashes that will ended in the water. I thing that this sedimentation could be affecting to the benthic community, but I would like to have some value of this sedimentation.

So if you only want to know about sediment settling down from above and not sediment generated in the lower water column or on the sea bed, you could simply install some buoyant tethered collectors.

You may visit English et al 1997 on coastal habitat and resources survey methods. Sediment measurement is covered in one section. Good luck