When using RTK or differential GPS technologies, considering the Base has a clear unobstructed view, what view of the sky is required to be able to get an accurate fix with the rover when is moving (boat on water)?
General rules would be (1) chose a base station that is as close as possible, and (2) whatever you do, make sure you use a receiver and a base station that both do GPS and GLONASS!
With GPS + GLONASS, you have a MUCH better chance of having enough satellites shared between rover and base station.
I have no experience on aquatic locations, but using a high-grade antenna (Trimble Zephyr 2, etc.) on your unit can reduce multipaths in a canyon and improve the solution.
For Trimble Real-Time Kinematic (RTK) and rovers a minimum of 4 satellites must be tracked to obtain RTK coordinate solutions. With 5 common satellites necessary for RTK initialization.
The Global Position System (GPS) is designed so at least 5 satellites are above the horizon at all times, but there can be 8 or more during some times of the day. See here for more information on this topic.
It is difficult to answer 'what view of the sky is required?, but in the most basic sense enough so that the base and rovers can pick up at least 4 satellites. The amount of sky required may differ depending on where you are in the world or what time of day it is.
There is work being done in Hells Canyon, ID which is a very deep canyon (North America's deepest river gorge) that uses RTK GPS as part of their setup when collecting bathymetric/topographic data with multibeam and single beam sonar. Despite the depth of the canyon in many places it is fairly open which allows the ability to obtain signals from 4 satellites.
In contrast the work done in the Grand Canyon by the Grand Canyon Monitoring and Research Center (GCMRC) is unable to leverage RTK GPS because of the lack of satellite coverage in the deep canyon. The sheer cliffs and relatively narrow nature of the Grand Canyon is perhaps more impactful on RTK than how deep it is. To gain more insight for how they overcome these limitations to collect topographic data it may be worthwhile for you to visit the GCMRC website here.
You can use mission planning tools where for a defined location you can input obstructions using bearing and elevation. You will then be able to see how these obstructions affect satellite availability and the GDOP (Geometric Dilution Of Precision). GDOP value is very important to know if the geometry of the visible satellites is good to achieve a reliable position.
Google "Trimble mission planning" for the software and make sure you download the latest almanac.