I use a Garmin etrex 200 for this purpose. Simple, reliable and fairly cheap.
It has got an inbuilt map but that is mainly for checking waypoint positions.
For plotting l then use the ipad with navionics.
In some ways the etrex is better as its designed to work in all conditions ie wet.
Hope that helps.

You may be interested in this.

The ZTE T55 (Telstra Tough 3) is a waterproof, dust proof, drop proof phone with Blue Tick technology giving it extended range in rural and remote areas.

It is the only phone I know of that has a stand alone GPS incorporated. When the GPS is switched on it pulls up the satellite page with 5 bars showing signal strength/quality and once at least 4 are acquired will go to the position page which can be set for decimal or degrees display.

It will give you a position in 60 seconds which rivals most dedicated hand held GPS units. It does not have a plotter or maps and does not store way points or routes but it does give you a lat/long position in 60 seconds.

I had to buy three to get two, one for myself and one for my mate. It is new and unused. I guess they were factory surplus. Is is unlocked but needs a $2 sim in it for it to function.

I am using mine to have a longer range water proof phone on board with GPS function which will be included in the "grab bag" in case of an abandon ship scenario.

There are plenty of used ones for sale on Ebay but this one is new and unused and comes with 240v charger and USB cord so therefore can be charged on board if you have a USB outlet. I am selling it for $100 plus postage.

www.ztemobiles.com.au/T55.htm

It is item 8 in tools. If yours is a T55 a it might not have it.

Kankama said..
I can use a phone GPS app but they aren't waterproof.

Lots of waterproof phones out there, just need to search for them. Or a good quality waterproof case on existing phone.
I've got a Samsung Galaxy S5 because it also had the magnetometer to run the Vavauud wind meter but there are a few newer Samsung models as well . Sony xperia Z range is another one and even my old Motorola Defy was waterproof.

My S5 is already water proof to about 0.5 mtr possibly more but was a bit slippery for the boat so I got a Lifeproof Fre case for it. Now it can take a few knocks in the cockpit and should be good for a 2mtr dip but only had it tested at around 1 mtr at the ramp when dropping the rudders and the phone slipped out of my shirt pocket. Had it tested for about 10 minutes while struggling to find it with my foot on the muddy bottom. If I was to buy another one I'd be looking for a yellow or red case, anything but black but something with built in flotation would be even more nicer.