If this is your first installation then you'll need to install some dependencies, as follows:
sudo apt-get -y install cmake gettext \
	gpsd gpsd-clients libgps-dev wx-common \
	 libwxgtk3.0-dev libgtk2.0-dev \
	wx3.0-headers  libbz2-dev libtinyxml-dev \
	portaudio19-dev  libcurl4-openssl-dev  libcairo2-dev

$ sudo dpkg -i opencpn_4.2.0-1_armhf_full.deb

Add a couple of lines to the /boot/config.txt file; use any text editor such as vi or leafpad. 
These changes will be effective following the next reboot and are required to resolve problems that the Pi/armhf has with vector charts.
$ vi /boot/config.txt
and insert the following lines (under existing framebuffer entries for convenience)
     framebuffer_depth=32
     framebuffer_ignore_alpha=1
you may find it useful to edit your ~/.opencpn.opencpn.conf file and add values for MEMCacheLimit and/or NCacheLimit .. see opencpn.org for details.




Refer to the opencpn.org if you want to build the latest OpenCPN for yourself ,  however, the following are the instructions that I use. Copy and paste the information, don't run it as a script!
 
 
########################################################
#       Install opencpn on Raspberry PI
#      12 Feb 16  by opencpn@agurney.com 
#      see http://opencpn.org/ocpn/compiling_source_linux    
########################################################################
#    2016-02-09-raspbian-jessie 
########################################################"

sudo raspi-config

   # Expand filesystem
   # Change the pi user's password from the default (raspberry)
   # Boot options (B4 autologon as PI with desktop)
   # Advanced -> Enable GL driver
   # ... anything else, e.g. overscan, overclock
   # REBOOT

# Install OpenCPN dependencies"

cd ~
sudo apt-get -y update
sudo apt-get -y upgrade
sudo apt-get -y install cmake gettext \
	gpsd gpsd-clients libgps-dev wx-common \
	 libwxgtk3.0-dev libgtk2.0-dev \
	wx3.0-headers  libbz2-dev libtinyxml-dev \
	portaudio19-dev  libcurl4-openssl-dev  libcairo2-dev

	
	
	
# retrieve OpenCPN source and build it

cd ~
git clone git://github.com/OpenCPN/OpenCPN.git

to update an existing local copy
cd opencpn
git pull



cd ~/.OpenCPN/
mkdir build
cd build

# With basic map only:
cmake -DBUNDLE_GSHHS=CRUDE ../


# If you want everything - DOCS, MAPS, TIDES then use this:
# needs https://sourceforge.net/projects/opencpnplugins/files/opencpn_packaging_data/
# see http://opencpn.org/ocpn/Modularized_Packaging 
# could use FULL rather than HIGH but it's a lot larger
cmake -DBUNDLE_DOCS=ON -DBUNDLE_TCDATA=ON -DBUNDLE_GSHHS=HIGH ../


# cmake -j3 should spread load across cores, but doesn't seem to work
make 

sudo make install

# might as well create a .deb packagae so you can restore without having to rebuild
sudo make package

#  now save the package so it can be used as a backup
... export the file (cp, FTP) ~/.OpenCPN/build/opencpn_4.2.0-1_armhf.deb (or whatever it's called) to external storage


# Add a couple of lines to the /boot/config.txt file; 
# use any text editor such as vi or leafpad.
# These changes will be effective following the next reboot and 
# are required to resolve problems that the Pi/armhf has with vector charts. 

sudo vi /boot/config.txt
   framebuffer_depth=32
   framebuffer_ignore_alpha=1

Reboot and you should be good to go
Next, create a directory and copy your charts, for example

mkdir /usr/local/share/charts
Free charts for the US are available from NOAA, You'll have to purchase charts for the rest of the world or scan your own; refer to opencpn.org for supported formats.
If, like me, you sail the West coast of Scotland I can recommend Antares charts .