I have downloaded mine into google earth, quite easy once the coords are converted.

 IF you have a Garmin, Homeport seems to do the job. You can also import GPS coords without owning a garmin unit however unless you have paid for a garmin map you will only see the points on their worldwide map which has very little detail ie, no contour lines.

 However, freeware version is limited in how many marks you can upload. Don't know how many exactly, but the 770-odd from the Lowrance was too many. I bought the paid version, and now have some 28,000 on there from my pro days.

Insight Planner is brilliant as you can download your marks and tracks along with the maps from your Navionics card so you can overlay your marks over the maps.

You can also create new marks and routes on the PC and upload them to your plotter.

Only downside is that it's a bit clunky and slow but that may be due to me having an entry kevel laptop.

 i use navplanner mostly , works great but it struggles when theres to many marks it freezes alot, but that could also be this old laptop,

i was given a huge file but it wouldnt load in there, but i manged a way to convert the marks and load them into google earth, you can get the addons KMZ files that show bathemetry ,contour lines etc 

export waypoints&tracks onto card, copy file onto computer, download gpsbabel, choose input as the file type you exported it as (simrad uses lorwance usd), change output to GPX, press convert, open google earth>tools>gps, browse to GPX file you created and you can visually see all the marks.
if you want to overlay the chart then you need insight planner.

Gpsfishingmaps for iPhone. Free version works well enough and takes gpx and klm files Easy gps for a free version for pc. Expert gps costs about $30 from memory and a heap easier to use. Added bonus of doing file type conversions on most gps breeds for ya. Or google earth as mentioned. Easiest and best and can use on phone and pc if only being used as a reference.For ur hds, you should be able to export direct to .gpx file type and that will work in earth

 Had 28, 164 from pro days. 880 from amateur days. merging is very simple, made one big file. Then nutted out how to break down by area. You can sort by lat or long, among other options. I sorted by Lat, and broke the coast from Dongara to Exmouth down into areas, heeding that I needed well under 10,000 for each file. So, for example, the southermost area went from Dongara to just north of Lucky bay. The northern end went from  26 deg S to Exmouth. The bit in the middle was my most heavily populated, so I broke that down into four areas, wth lots of overlap. Then, just for the hell of it, I sorted by Long, and found a point well north of Kalbarri. Took everything east of that point, which effectively gives me one big shallows and near shore deep map. It took a bit of getting my head around, but if you ACTUALLY READ THE HELP FILES ( shouting at myself here ) the process is straightforward.